diff --git a/InvenTree/InvenTree/filters.py b/InvenTree/InvenTree/filters.py
index cd1b769646..94e6e1765b 100644
--- a/InvenTree/InvenTree/filters.py
+++ b/InvenTree/InvenTree/filters.py
@@ -34,18 +34,47 @@ class InvenTreeOrderingFilter(OrderingFilter):
             Ordering fields should be mapped to separate fields
             """
 
-            for idx, field in enumerate(ordering):
+            ordering_initial = ordering
+            ordering = []
 
-                reverse = False
+            for field in ordering_initial:
+                
+                reverse = field.startswith('-')
 
-                if field.startswith('-'):
+                if reverse:
                     field = field[1:]
-                    reverse = True
 
+                # Are aliases defined for this field?
                 if field in aliases:
-                    ordering[idx] = aliases[field]
+                    alias = aliases[field]
+                else:
+                    alias = field
 
+                """
+                Potentially, a single field could be "aliased" to multiple field,
+                
+                (For example to enforce a particular ordering sequence)
+
+                e.g. to filter first by the integer value...
+
+                ordering_field_aliases = {
+                    "reference": ["integer_ref", "reference"]
+                }
+
+                """
+
+                if type(alias) is str:
+                    alias = [alias]
+                elif type(alias) in [list, tuple]:
+                    pass
+                else:
+                    # Unsupported alias type
+                    continue
+
+                for a in alias:
                     if reverse:
-                        ordering[idx] = '-' + ordering[idx]
+                        a = '-' + a
+
+                    ordering.append(a)
 
         return ordering
diff --git a/InvenTree/InvenTree/forms.py b/InvenTree/InvenTree/forms.py
index 8b4b87637c..cd431a5f93 100644
--- a/InvenTree/InvenTree/forms.py
+++ b/InvenTree/InvenTree/forms.py
@@ -4,10 +4,12 @@ Helper forms which subclass Django forms to provide additional functionality
 
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
+import logging
 
 from django.utils.translation import ugettext_lazy as _
 from django import forms
-from django.contrib.auth.models import User
+from django.contrib.auth.models import User, Group
+from django.conf import settings
 
 from crispy_forms.helper import FormHelper
 from crispy_forms.layout import Layout, Field
@@ -20,6 +22,8 @@ from allauth.socialaccount.adapter import DefaultSocialAccountAdapter
 from part.models import PartCategory
 from common.models import InvenTreeSetting
 
+logger = logging.getLogger('inventree')
+
 
 class HelperForm(forms.ModelForm):
     """ Provides simple integration of crispy_forms extension. """
@@ -223,11 +227,11 @@ class CustomSignupForm(SignupForm):
         # check for two mail fields
         if InvenTreeSetting.get_setting('LOGIN_SIGNUP_MAIL_TWICE'):
             self.fields["email2"] = forms.EmailField(
-                label=_("E-mail (again)"),
+                label=_("Email (again)"),
                 widget=forms.TextInput(
                     attrs={
                         "type": "email",
-                        "placeholder": _("E-mail address confirmation"),
+                        "placeholder": _("Email address confirmation"),
                     }
                 ),
             )
@@ -256,11 +260,23 @@ class RegistratonMixin:
     """
     Mixin to check if registration should be enabled
     """
-    def is_open_for_signup(self, request):
-        if InvenTreeSetting.get_setting('EMAIL_HOST', None) and InvenTreeSetting.get_setting('LOGIN_ENABLE_REG', True):
-            return super().is_open_for_signup(request)
+    def is_open_for_signup(self, request, *args, **kwargs):
+        if settings.EMAIL_HOST and InvenTreeSetting.get_setting('LOGIN_ENABLE_REG', True):
+            return super().is_open_for_signup(request, *args, **kwargs)
         return False
 
+    def save_user(self, request, user, form, commit=True):
+        user = super().save_user(request, user, form, commit=commit)
+        start_group = InvenTreeSetting.get_setting('SIGNUP_GROUP')
+        if start_group:
+            try:
+                group = Group.objects.get(id=start_group)
+                user.groups.add(group)
+            except Group.DoesNotExist:
+                logger.error('The setting `SIGNUP_GROUP` contains an non existant group', start_group)
+        user.save()
+        return user
+
 
 class CustomAccountAdapter(RegistratonMixin, DefaultAccountAdapter):
     """
@@ -268,7 +284,7 @@ class CustomAccountAdapter(RegistratonMixin, DefaultAccountAdapter):
     """
     def send_mail(self, template_prefix, email, context):
         """only send mail if backend configured"""
-        if InvenTreeSetting.get_setting('EMAIL_HOST', None):
+        if settings.EMAIL_HOST:
             return super().send_mail(template_prefix, email, context)
         return False
 
diff --git a/InvenTree/InvenTree/models.py b/InvenTree/InvenTree/models.py
index 2ca179bb40..0f8350f84f 100644
--- a/InvenTree/InvenTree/models.py
+++ b/InvenTree/InvenTree/models.py
@@ -4,6 +4,7 @@ Generic models which provide extra functionality over base Django model types.
 
 from __future__ import unicode_literals
 
+import re
 import os
 import logging
 
@@ -43,6 +44,48 @@ def rename_attachment(instance, filename):
     return os.path.join(instance.getSubdir(), filename)
 
 
+class ReferenceIndexingMixin(models.Model):
+    """
+    A mixin for keeping track of numerical copies of the "reference" field.
+
+    Here, we attempt to convert a "reference" field value (char) to an integer,
+    for performing fast natural sorting.
+
+    This requires extra database space (due to the extra table column),
+    but is required as not all supported database backends provide equivalent casting.
+
+    This mixin adds a field named 'reference_int'.
+
+    - If the 'reference' field can be cast to an integer, it is stored here
+    - If the 'reference' field *starts* with an integer, it is stored here
+    - Otherwise, we store zero
+    """
+
+    class Meta:
+        abstract = True
+
+    def rebuild_reference_field(self):
+
+        reference = getattr(self, 'reference', '')
+
+        # Default value if we cannot convert to an integer
+        ref_int = 0
+
+        # Look at the start of the string - can it be "integerized"?
+        result = re.match(r"^(\d+)", reference)
+
+        if result and len(result.groups()) == 1:
+            ref = result.groups()[0]
+            try:
+                ref_int = int(ref)
+            except:
+                ref_int = 0
+
+        self.reference_int = ref_int
+
+    reference_int = models.IntegerField(default=0)
+
+
 class InvenTreeAttachment(models.Model):
     """ Provides an abstracted class for managing file attachments.
 
diff --git a/InvenTree/InvenTree/settings.py b/InvenTree/InvenTree/settings.py
index a07324ec84..2095cab533 100644
--- a/InvenTree/InvenTree/settings.py
+++ b/InvenTree/InvenTree/settings.py
@@ -385,39 +385,6 @@ Q_CLUSTER = {
     'sync': False,
 }
 
-# Markdownx configuration
-# Ref: https://neutronx.github.io/django-markdownx/customization/
-MARKDOWNX_MEDIA_PATH = datetime.now().strftime('markdownx/%Y/%m/%d')
-
-# Markdownify configuration
-# Ref: https://django-markdownify.readthedocs.io/en/latest/settings.html
-
-MARKDOWNIFY_WHITELIST_TAGS = [
-    'a',
-    'abbr',
-    'b',
-    'blockquote',
-    'em',
-    'h1', 'h2', 'h3',
-    'i',
-    'img',
-    'li',
-    'ol',
-    'p',
-    'strong',
-    'ul'
-]
-
-MARKDOWNIFY_WHITELIST_ATTRS = [
-    'href',
-    'src',
-    'alt',
-]
-
-MARKDOWNIFY_BLEACH = False
-
-DATABASES = {}
-
 """
 Configure the database backend based on the user-specified values.
 
@@ -484,7 +451,47 @@ logger.info(f"DB_ENGINE: {db_engine}")
 logger.info(f"DB_NAME: {db_name}")
 logger.info(f"DB_HOST: {db_host}")
 
-DATABASES['default'] = db_config
+"""
+In addition to base-level database configuration, we may wish to specify specific options to the database backend
+Ref: https://docs.djangoproject.com/en/3.2/ref/settings/#std:setting-OPTIONS
+"""
+
+# 'OPTIONS' or 'options' can be specified in config.yaml
+db_options = db_config.get('OPTIONS', db_config.get('options', {}))
+
+# Specific options for postgres backend
+if 'postgres' in db_engine:
+    from psycopg2.extensions import ISOLATION_LEVEL_READ_COMMITTED, ISOLATION_LEVEL_SERIALIZABLE
+
+    # Connection timeout
+    if 'connect_timeout' not in db_options:
+        db_options['connect_timeout'] = int(os.getenv('INVENTREE_DB_TIMEOUT', 2))
+
+    # Postgres's default isolation level is Read Committed which is
+    # normally fine, but most developers think the database server is
+    # actually going to do Serializable type checks on the queries to
+    # protect against simultaneous changes.
+    if 'isolation_level' not in db_options:
+        serializable = _is_true(os.getenv("PG_ISOLATION_SERIALIZABLE", "true"))
+        db_options['isolation_level'] = ISOLATION_LEVEL_SERIALIZABLE if serializable else ISOLATION_LEVEL_READ_COMMITTED
+
+# Specific options for MySql / MariaDB backend
+if 'mysql' in db_engine:
+    # TODO
+    pass
+
+# Specific options for sqlite backend
+if 'sqlite' in db_engine:
+    # TODO
+    pass
+
+# Provide OPTIONS dict back to the database configuration dict
+db_config['OPTIONS'] = db_options
+
+DATABASES = {
+    'default': db_config
+}
+
 
 CACHES = {
     'default': {
@@ -683,3 +690,34 @@ ACCOUNT_FORMS = {
 
 SOCIALACCOUNT_ADAPTER = 'InvenTree.forms.CustomSocialAccountAdapter'
 ACCOUNT_ADAPTER = 'InvenTree.forms.CustomAccountAdapter'
+
+# Markdownx configuration
+# Ref: https://neutronx.github.io/django-markdownx/customization/
+MARKDOWNX_MEDIA_PATH = datetime.now().strftime('markdownx/%Y/%m/%d')
+
+# Markdownify configuration
+# Ref: https://django-markdownify.readthedocs.io/en/latest/settings.html
+
+MARKDOWNIFY_WHITELIST_TAGS = [
+    'a',
+    'abbr',
+    'b',
+    'blockquote',
+    'em',
+    'h1', 'h2', 'h3',
+    'i',
+    'img',
+    'li',
+    'ol',
+    'p',
+    'strong',
+    'ul'
+]
+
+MARKDOWNIFY_WHITELIST_ATTRS = [
+    'href',
+    'src',
+    'alt',
+]
+
+MARKDOWNIFY_BLEACH = False
diff --git a/InvenTree/build/admin.py b/InvenTree/build/admin.py
index 86592c7a81..a5ad838660 100644
--- a/InvenTree/build/admin.py
+++ b/InvenTree/build/admin.py
@@ -9,6 +9,10 @@ from .models import Build, BuildItem
 
 class BuildAdmin(ImportExportModelAdmin):
 
+    exclude = [
+        'reference_int',
+    ]
+
     list_display = (
         'reference',
         'title',
diff --git a/InvenTree/build/api.py b/InvenTree/build/api.py
index dc2e81b978..cfc2cab16f 100644
--- a/InvenTree/build/api.py
+++ b/InvenTree/build/api.py
@@ -15,6 +15,7 @@ from django_filters import rest_framework as rest_filters
 
 from InvenTree.api import AttachmentMixin
 from InvenTree.helpers import str2bool, isNull
+from InvenTree.filters import InvenTreeOrderingFilter
 from InvenTree.status_codes import BuildStatus
 
 from .models import Build, BuildItem, BuildOrderAttachment
@@ -66,7 +67,7 @@ class BuildList(generics.ListCreateAPIView):
     filter_backends = [
         DjangoFilterBackend,
         filters.SearchFilter,
-        filters.OrderingFilter,
+        InvenTreeOrderingFilter,
     ]
 
     ordering_fields = [
@@ -81,6 +82,10 @@ class BuildList(generics.ListCreateAPIView):
         'responsible',
     ]
 
+    ordering_field_aliases = {
+        'reference': ['reference_int', 'reference'],
+    }
+
     search_fields = [
         'reference',
         'part__name',
diff --git a/InvenTree/build/migrations/0031_build_reference_int.py b/InvenTree/build/migrations/0031_build_reference_int.py
new file mode 100644
index 0000000000..c7fc2c16cc
--- /dev/null
+++ b/InvenTree/build/migrations/0031_build_reference_int.py
@@ -0,0 +1,18 @@
+# Generated by Django 3.2.5 on 2021-10-14 06:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0030_alter_build_reference'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='reference_int',
+            field=models.IntegerField(default=0),
+        ),
+    ]
diff --git a/InvenTree/build/migrations/0032_auto_20211014_0632.py b/InvenTree/build/migrations/0032_auto_20211014_0632.py
new file mode 100644
index 0000000000..3dac2b30c6
--- /dev/null
+++ b/InvenTree/build/migrations/0032_auto_20211014_0632.py
@@ -0,0 +1,50 @@
+# Generated by Django 3.2.5 on 2021-10-14 06:32
+
+import re
+
+from django.db import migrations
+
+
+def build_refs(apps, schema_editor):
+    """
+    Rebuild the integer "reference fields" for existing Build objects
+    """
+
+    BuildOrder = apps.get_model('build', 'build')
+
+    for build in BuildOrder.objects.all():
+
+        ref = 0
+
+        result = re.match(r"^(\d+)", build.reference)
+
+        if result and len(result.groups()) == 1:
+            try:
+                ref = int(result.groups()[0])
+            except:
+                ref = 0
+
+        build.reference_int = ref
+        build.save()
+
+def unbuild_refs(apps, schema_editor):
+    """
+    Provided only for reverse migration compatibility
+    """
+    pass
+
+
+class Migration(migrations.Migration):
+
+    atomic = False
+
+    dependencies = [
+        ('build', '0031_build_reference_int'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            build_refs,
+            reverse_code=unbuild_refs
+        )
+    ]
diff --git a/InvenTree/build/models.py b/InvenTree/build/models.py
index 994d060585..403b3a9430 100644
--- a/InvenTree/build/models.py
+++ b/InvenTree/build/models.py
@@ -28,7 +28,7 @@ from mptt.exceptions import InvalidMove
 from InvenTree.status_codes import BuildStatus, StockStatus, StockHistoryCode
 from InvenTree.helpers import increment, getSetting, normalize, MakeBarcode
 from InvenTree.validators import validate_build_order_reference
-from InvenTree.models import InvenTreeAttachment
+from InvenTree.models import InvenTreeAttachment, ReferenceIndexingMixin
 
 import common.models
 
@@ -69,7 +69,7 @@ def get_next_build_number():
     return reference
 
 
-class Build(MPTTModel):
+class Build(MPTTModel, ReferenceIndexingMixin):
     """ A Build object organises the creation of new StockItem objects from other existing StockItem objects.
 
     Attributes:
@@ -108,6 +108,8 @@ class Build(MPTTModel):
 
     def save(self, *args, **kwargs):
 
+        self.rebuild_reference_field()
+
         try:
             super().save(*args, **kwargs)
         except InvalidMove:
diff --git a/InvenTree/build/test_build.py b/InvenTree/build/test_build.py
index 31e4cf1822..f8c381f224 100644
--- a/InvenTree/build/test_build.py
+++ b/InvenTree/build/test_build.py
@@ -118,6 +118,26 @@ class BuildTest(TestCase):
 
         self.stock_3_1 = StockItem.objects.create(part=self.sub_part_3, quantity=1000)
 
+    def test_ref_int(self):
+        """
+        Test the "integer reference" field used for natural sorting
+        """
+
+        for ii in range(10):
+            build = Build(
+                reference=f"{ii}_abcde",
+                quantity=1,
+                part=self.assembly,
+                title="Making some parts"
+            )
+
+            self.assertEqual(build.reference_int, 0)
+
+            build.save()
+
+            # After saving, the integer reference should have been updated
+            self.assertEqual(build.reference_int, ii)
+
     def test_init(self):
         # Perform some basic tests before we start the ball rolling
 
diff --git a/InvenTree/common/models.py b/InvenTree/common/models.py
index d4f26af739..81fee4ff65 100644
--- a/InvenTree/common/models.py
+++ b/InvenTree/common/models.py
@@ -11,7 +11,7 @@ import decimal
 import math
 
 from django.db import models, transaction
-from django.contrib.auth.models import User
+from django.contrib.auth.models import User, Group
 from django.db.utils import IntegrityError, OperationalError
 from django.conf import settings
 
@@ -182,12 +182,9 @@ class BaseInvenTreeSetting(models.Model):
         else:
             choices = None
 
-        """
-        TODO:
-        if type(choices) is function:
+        if callable(choices):
             # Evaluate the function (we expect it will return a list of tuples...)
             return choices()
-        """
 
         return choices
 
@@ -479,6 +476,11 @@ class BaseInvenTreeSetting(models.Model):
         return value
 
 
+def settings_group_options():
+    """build up group tuple for settings based on gour choices"""
+    return [('', _('No group')), *[(str(a.id), str(a)) for a in Group.objects.all()]]
+
+
 class InvenTreeSetting(BaseInvenTreeSetting):
     """
     An InvenTreeSetting object is a key:value pair used for storing
@@ -822,7 +824,7 @@ class InvenTreeSetting(BaseInvenTreeSetting):
             'validator': bool,
         },
         'LOGIN_MAIL_REQUIRED': {
-            'name': _('E-Mail required'),
+            'name': _('Email required'),
             'description': _('Require user to supply mail on signup'),
             'default': False,
             'validator': bool,
@@ -845,6 +847,12 @@ class InvenTreeSetting(BaseInvenTreeSetting):
             'default': True,
             'validator': bool,
         },
+        'SIGNUP_GROUP': {
+            'name': _('Group on signup'),
+            'description': _('Group new user are asigned on registration'),
+            'default': '',
+            'choices': settings_group_options
+        },
     }
 
     class Meta:
diff --git a/InvenTree/order/admin.py b/InvenTree/order/admin.py
index 54e91ed844..25b0922291 100644
--- a/InvenTree/order/admin.py
+++ b/InvenTree/order/admin.py
@@ -20,6 +20,10 @@ class PurchaseOrderLineItemInlineAdmin(admin.StackedInline):
 
 class PurchaseOrderAdmin(ImportExportModelAdmin):
 
+    exclude = [
+        'reference_int',
+    ]
+
     list_display = (
         'reference',
         'supplier',
@@ -41,6 +45,10 @@ class PurchaseOrderAdmin(ImportExportModelAdmin):
 
 class SalesOrderAdmin(ImportExportModelAdmin):
 
+    exclude = [
+        'reference_int',
+    ]
+
     list_display = (
         'reference',
         'customer',
diff --git a/InvenTree/order/api.py b/InvenTree/order/api.py
index 42ed28dff0..1b294d3e3e 100644
--- a/InvenTree/order/api.py
+++ b/InvenTree/order/api.py
@@ -151,9 +151,13 @@ class POList(generics.ListCreateAPIView):
     filter_backends = [
         rest_filters.DjangoFilterBackend,
         filters.SearchFilter,
-        filters.OrderingFilter,
+        InvenTreeOrderingFilter,
     ]
 
+    ordering_field_aliases = {
+        'reference': ['reference_int', 'reference'],
+    }
+
     filter_fields = [
         'supplier',
     ]
@@ -489,9 +493,13 @@ class SOList(generics.ListCreateAPIView):
     filter_backends = [
         rest_filters.DjangoFilterBackend,
         filters.SearchFilter,
-        filters.OrderingFilter,
+        InvenTreeOrderingFilter,
     ]
 
+    ordering_field_aliases = {
+        'reference': ['reference_int', 'reference'],
+    }
+
     filter_fields = [
         'customer',
     ]
diff --git a/InvenTree/order/migrations/0051_auto_20211014_0623.py b/InvenTree/order/migrations/0051_auto_20211014_0623.py
new file mode 100644
index 0000000000..20cc893dd2
--- /dev/null
+++ b/InvenTree/order/migrations/0051_auto_20211014_0623.py
@@ -0,0 +1,23 @@
+# Generated by Django 3.2.5 on 2021-10-14 06:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0050_alter_purchaseorderlineitem_destination'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='purchaseorder',
+            name='reference_int',
+            field=models.IntegerField(default=0),
+        ),
+        migrations.AddField(
+            model_name='salesorder',
+            name='reference_int',
+            field=models.IntegerField(default=0),
+        ),
+    ]
diff --git a/InvenTree/order/migrations/0052_auto_20211014_0631.py b/InvenTree/order/migrations/0052_auto_20211014_0631.py
new file mode 100644
index 0000000000..b400437d20
--- /dev/null
+++ b/InvenTree/order/migrations/0052_auto_20211014_0631.py
@@ -0,0 +1,66 @@
+# Generated by Django 3.2.5 on 2021-10-14 06:31
+
+import re
+
+from django.db import migrations
+
+def build_refs(apps, schema_editor):
+    """
+    Rebuild the integer "reference fields" for existing Build objects
+    """
+
+    PurchaseOrder = apps.get_model('order', 'purchaseorder')
+
+    for order in PurchaseOrder.objects.all():
+
+        ref = 0
+
+        result = re.match(r"^(\d+)", order.reference)
+
+        if result and len(result.groups()) == 1:
+            try:
+                ref = int(result.groups()[0])
+            except:
+                ref = 0
+
+        order.reference_int = ref
+        order.save()
+
+    SalesOrder = apps.get_model('order', 'salesorder')
+
+    for order in SalesOrder.objects.all():
+
+        ref = 0
+
+        result = re.match(r"^(\d+)", order.reference)
+
+        if result and len(result.groups()) == 1:
+            try:
+                ref = int(result.groups()[0])
+            except:
+                ref = 0
+
+        order.reference_int = ref
+        order.save()
+
+
+def unbuild_refs(apps, schema_editor):
+    """
+    Provided only for reverse migration compatibility
+    """
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0051_auto_20211014_0623'),
+    ]
+
+
+    operations = [
+        migrations.RunPython(
+            build_refs,
+            reverse_code=unbuild_refs
+        )
+    ]
diff --git a/InvenTree/order/models.py b/InvenTree/order/models.py
index 4ac8925259..0c45e3746a 100644
--- a/InvenTree/order/models.py
+++ b/InvenTree/order/models.py
@@ -28,7 +28,7 @@ from company.models import Company, SupplierPart
 from InvenTree.fields import InvenTreeModelMoneyField, RoundingDecimalField
 from InvenTree.helpers import decimal2string, increment, getSetting
 from InvenTree.status_codes import PurchaseOrderStatus, SalesOrderStatus, StockStatus, StockHistoryCode
-from InvenTree.models import InvenTreeAttachment
+from InvenTree.models import InvenTreeAttachment, ReferenceIndexingMixin
 
 
 def get_next_po_number():
@@ -89,7 +89,7 @@ def get_next_so_number():
     return reference
 
 
-class Order(models.Model):
+class Order(ReferenceIndexingMixin):
     """ Abstract model for an order.
 
     Instances of this class:
@@ -147,6 +147,9 @@ class Order(models.Model):
         return new_ref
 
     def save(self, *args, **kwargs):
+
+        self.rebuild_reference_field()
+
         if not self.creation_date:
             self.creation_date = datetime.now().date()
 
@@ -531,6 +534,12 @@ class SalesOrder(Order):
 
         return queryset
 
+    def save(self, *args, **kwargs):
+
+        self.rebuild_reference_field()
+
+        super().save(*args, **kwargs)
+
     def __str__(self):
 
         prefix = getSetting('SALESORDER_REFERENCE_PREFIX')
diff --git a/InvenTree/order/test_migrations.py b/InvenTree/order/test_migrations.py
new file mode 100644
index 0000000000..b7db1f1b70
--- /dev/null
+++ b/InvenTree/order/test_migrations.py
@@ -0,0 +1,59 @@
+"""
+Unit tests for the 'order' model data migrations
+"""
+
+from django_test_migrations.contrib.unittest_case import MigratorTestCase
+
+from InvenTree import helpers
+
+
+class TestForwardMigrations(MigratorTestCase):
+    """
+    Test entire schema migration
+    """
+
+    migrate_from = ('order', helpers.getOldestMigrationFile('order'))
+    migrate_to = ('order', helpers.getNewestMigrationFile('order'))
+
+    def prepare(self):
+        """
+        Create initial data set
+        """
+
+        # Create a purchase order from a supplier
+        Company = self.old_state.apps.get_model('company', 'company')
+
+        supplier = Company.objects.create(
+            name='Supplier A',
+            description='A great supplier!',
+            is_supplier=True
+        )
+
+        PurchaseOrder = self.old_state.apps.get_model('order', 'purchaseorder')
+
+        # Create some orders
+        for ii in range(10):
+
+            order = PurchaseOrder.objects.create(
+                supplier=supplier,
+                reference=f"{ii}-abcde",
+                description="Just a test order"
+            )
+
+            # Initially, the 'reference_int' field is unavailable
+            with self.assertRaises(AttributeError):
+                print(order.reference_int)
+
+    def test_ref_field(self):
+        """
+        Test that the 'reference_int' field has been created and is filled out correctly
+        """
+
+        PurchaseOrder = self.new_state.apps.get_model('order', 'purchaseorder')
+
+        for ii in range(10):
+
+            order = PurchaseOrder.objects.get(reference=f"{ii}-abcde")
+
+            # The integer reference field must have been correctly updated
+        self.assertEqual(order.reference_int, ii)
diff --git a/InvenTree/part/api.py b/InvenTree/part/api.py
index dccc2f9ac1..4dd9841332 100644
--- a/InvenTree/part/api.py
+++ b/InvenTree/part/api.py
@@ -814,6 +814,27 @@ class PartList(generics.ListCreateAPIView):
             except (ValueError, Part.DoesNotExist):
                 pass
 
+        # Exclude specific part ID values?
+        exclude_id = []
+
+        for key in ['exclude_id', 'exclude_id[]']:
+            if key in params:
+                exclude_id += params.getlist(key, [])
+
+        if exclude_id:
+
+            id_values = []
+
+            for val in exclude_id:
+                try:
+                    # pk values must be integer castable
+                    val = int(val)
+                    id_values.append(val)
+                except ValueError:
+                    pass
+            
+            queryset = queryset.exclude(pk__in=id_values)
+
         # Exclude part variant tree?
         exclude_tree = params.get('exclude_tree', None)
 
diff --git a/InvenTree/templates/InvenTree/settings/login.html b/InvenTree/templates/InvenTree/settings/login.html
index 289f87a3c9..7ffebee91e 100644
--- a/InvenTree/templates/InvenTree/settings/login.html
+++ b/InvenTree/templates/InvenTree/settings/login.html
@@ -14,7 +14,6 @@
 <table class='table table-striped table-condensed'>
     {% include "InvenTree/settings/header.html" %}
     <tbody>
-        {% include "InvenTree/settings/setting.html" with key="LOGIN_ENABLE_REG" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_ENABLE_SSO" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_ENABLE_PWD_FORGOT" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_MAIL_REQUIRED" icon="fa-info-circle" %}
@@ -22,9 +21,11 @@
             <td>{% trans 'Signup' %}</td>
             <td colspan='4'></td>
         </tr>
+        {% include "InvenTree/settings/setting.html" with key="LOGIN_ENABLE_REG" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_SIGNUP_MAIL_TWICE" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_SIGNUP_PWD_TWICE" icon="fa-info-circle" %}
         {% include "InvenTree/settings/setting.html" with key="LOGIN_SIGNUP_SSO_AUTO" icon="fa-info-circle" %}
+        {% include "InvenTree/settings/setting.html" with key="SIGNUP_GROUP" %}
     </tbody>
 </table>
 
diff --git a/InvenTree/templates/InvenTree/settings/user.html b/InvenTree/templates/InvenTree/settings/user.html
index 569b218b43..d6cbf998a7 100644
--- a/InvenTree/templates/InvenTree/settings/user.html
+++ b/InvenTree/templates/InvenTree/settings/user.html
@@ -39,12 +39,12 @@
 </table>
 
 <div class='panel-heading'>
-    <h4>{% trans "E-Mail" %}</h4>
+    <h4>{% trans "Email" %}</h4>
 </div>
 
 <div>
     {% if user.emailaddress_set.all %}
-    <p>{% trans 'The following e-mail addresses are associated with your account:' %}</p>
+    <p>{% trans 'The following email addresses are associated with your account:' %}</p>
 
     <form action="{% url 'account_email' %}" class="email_list" method="post">
     {% csrf_token %}
@@ -78,19 +78,19 @@
 
     {% else %}
     <p><strong>{% trans 'Warning:'%}</strong>
-        {% trans "You currently do not have any e-mail address set up. You should really add an e-mail address so you can receive notifications, reset your password, etc." %}
+        {% trans "You currently do not have any email address set up. You should really add an email address so you can receive notifications, reset your password, etc." %}
     </p>
 
     {% endif %}
 
     {% if can_add_email %}
         <br>
-        <h4>{% trans "Add E-mail Address" %}</h4>
+        <h4>{% trans "Add Email Address" %}</h4>
 
         <form method="post" action="{% url 'account_email' %}" class="add_email">
             {% csrf_token %}
             {{ add_email_form|crispy }}
-            <button class="btn btn-primary" name="action_add" type="submit">{% trans "Add E-mail" %}</button>
+            <button class="btn btn-primary" name="action_add" type="submit">{% trans "Add Email" %}</button>
         </form>
     {% endif %}
     <br>
@@ -220,7 +220,7 @@
 
 {% block js_ready %}
 (function() {
-  var message = "{% trans 'Do you really want to remove the selected e-mail address?' %}";
+  var message = "{% trans 'Do you really want to remove the selected email address?' %}";
   var actions = document.getElementsByName('action_remove');
   if (actions.length) {
     actions[0].addEventListener("click", function(e) {
diff --git a/InvenTree/templates/account/email_confirm.html b/InvenTree/templates/account/email_confirm.html
index 12b041f710..1bdd051fdc 100644
--- a/InvenTree/templates/account/email_confirm.html
+++ b/InvenTree/templates/account/email_confirm.html
@@ -3,17 +3,17 @@
 {% load i18n %}
 {% load account %}
 
-{% block head_title %}{% trans "Confirm E-mail Address" %}{% endblock %}
+{% block head_title %}{% trans "Confirm Email Address" %}{% endblock %}
 
 
 {% block content %}
-<h1>{% trans "Confirm E-mail Address" %}</h1>
+<h1>{% trans "Confirm Email Address" %}</h1>
 
 {% if confirmation %}
 
 {% user_display confirmation.email_address.user as user_display %}
 
-<p>{% blocktrans with confirmation.email_address.email as email %}Please confirm that <a href="mailto:{{ email }}">{{ email }}</a> is an e-mail address for user {{ user_display }}.{% endblocktrans %}</p>
+<p>{% blocktrans with confirmation.email_address.email as email %}Please confirm that <a href="mailto:{{ email }}">{{ email }}</a> is an email address for user {{ user_display }}.{% endblocktrans %}</p>
 
 <form method="post" action="{% url 'account_confirm_email' confirmation.key %}">
 {% csrf_token %}
@@ -24,7 +24,7 @@
 
 {% url 'account_email' as email_url %}
 
-<p>{% blocktrans %}This e-mail confirmation link expired or is invalid. Please <a href="{{ email_url }}">issue a new e-mail confirmation request</a>.{% endblocktrans %}</p>
+<p>{% blocktrans %}This email confirmation link expired or is invalid. Please <a href="{{ email_url }}">issue a new email confirmation request</a>.{% endblocktrans %}</p>
 
 {% endif %}
 
diff --git a/InvenTree/templates/account/password_reset.html b/InvenTree/templates/account/password_reset.html
index 2cfb45a716..1eeb5c6179 100644
--- a/InvenTree/templates/account/password_reset.html
+++ b/InvenTree/templates/account/password_reset.html
@@ -15,7 +15,7 @@
     {% endif %}
 
     {% if mail_conf and enable_pwd_forgot %}
-    <p>{% trans "Forgotten your password? Enter your e-mail address below, and we'll send you an e-mail allowing you to reset it." %}</p>
+    <p>{% trans "Forgotten your password? Enter your email address below, and we'll send you an email allowing you to reset it." %}</p>
 
     <form method="POST" action="{% url 'account_reset_password' %}" class="password_reset">
         {% csrf_token %}
diff --git a/InvenTree/templates/js/translated/bom.js b/InvenTree/templates/js/translated/bom.js
index 7cab63153b..89ae428bcd 100644
--- a/InvenTree/templates/js/translated/bom.js
+++ b/InvenTree/templates/js/translated/bom.js
@@ -157,6 +157,19 @@ function bomSubstitutesDialog(bom_item_id, substitutes, options={}) {
         }
     }
 
+    // Extract a list of all existing "substitute" id values
+    function getSubstituteIdValues(modal) {
+
+        var id_values = [];
+
+        $(modal).find('.substitute-row').each(function(el) {
+            var part = $(this).attr('part');
+            id_values.push(part);
+        });
+
+        return id_values;
+    }
+
     function renderSubstituteRow(substitute) {
 
         var pk = substitute.pk;
@@ -171,7 +184,7 @@ function bomSubstitutesDialog(bom_item_id, substitutes, options={}) {
 
         // Render a single row
         var html = `
-        <tr id='substitute-row-${pk}' class='substitute-row'>
+        <tr id='substitute-row-${pk}' class='substitute-row' part='${substitute.part}'>
             <td id='part-${pk}'>
                 <a href='/part/${part.pk}/'>
                     ${thumb} ${part.full_name}
@@ -246,6 +259,21 @@ function bomSubstitutesDialog(bom_item_id, substitutes, options={}) {
             },
             part: {
                 required: false,
+                adjustFilters: function(query, opts) {
+
+                    var subs = getSubstituteIdValues(opts.modal);
+
+                    // Also exclude the "master" part (if provided)
+                    if (options.sub_part) {
+                        subs.push(options.sub_part);
+                    }
+
+                    if (subs.length > 0) {
+                        query.exclude_id = subs;
+                    }
+
+                    return query;
+                }
             },
         },
         preFormContent: html,
@@ -801,6 +829,7 @@ function loadBomTable(table, options) {
                 subs,
                 {
                     table: table,
+                    sub_part: row.sub_part,
                 }
             );
         });
diff --git a/InvenTree/templates/js/translated/forms.js b/InvenTree/templates/js/translated/forms.js
index fecf5f1bd6..2483263219 100644
--- a/InvenTree/templates/js/translated/forms.js
+++ b/InvenTree/templates/js/translated/forms.js
@@ -1349,7 +1349,7 @@ function initializeRelatedField(field, fields, options) {
                 
                 // Allow custom run-time filter augmentation
                 if ('adjustFilters' in field) {
-                    query = field.adjustFilters(query);
+                    query = field.adjustFilters(query, options);
                 }
 
                 return query;
diff --git a/requirements.txt b/requirements.txt
index 9fa4149aba..b9f1dfd692 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,41 +1,39 @@
-# Django framework
+# Please keep this list sorted
 Django==3.2.5                   # Django package
-gunicorn>=20.1.0                # Gunicorn web server
-
-pillow==8.3.2                   # Image manipulation
-djangorestframework==3.12.4     # DRF framework
-django-cors-headers==3.2.0      # CORS headers extension for DRF
-django-filter==2.4.0            # Extended filtering options
-django-mptt==0.11.0             # Modified Preorder Tree Traversal
-django-sql-utils==0.5.0         # Advanced query annotation / aggregation
-django-markdownx==3.0.1         # Markdown form fields
-django-markdownify==0.8.0       # Markdown rendering
+certifi                         # Certifi is (most likely) installed through one of the requirements above
 coreapi==2.3.0                  # API documentation
-pygments==2.7.4                 # Syntax highlighting
-django-crispy-forms==1.11.2     # Form helpers
-django-import-export==2.5.0     # Data import / export for admin interface
-tablib[xls,xlsx,yaml]           # Support for XLS and XLSX formats
-django-cleanup==5.1.0           # Manage deletion of old / unused uploaded files
-flake8==3.8.3                   # PEP checking
-pep8-naming==0.11.1             # PEP naming convention extension
 coverage==5.3                   # Unit test coverage
 coveralls==2.1.2                # Coveralls linking (for Travis)
-rapidfuzz==0.7.6                # Fuzzy string matching
-django-stdimage==5.1.1          # Advanced ImageField management
-weasyprint==52.5                # PDF generation library (Note: in the future need to update to 53)
-django-weasyprint==1.0.1        # django weasyprint integration
-django-debug-toolbar==2.2       # Debug / profiling toolbar
+cryptography==3.4.8             # Cryptography support
 django-admin-shell==0.1.2       # Python shell for the admin interface
-py-moneyed==0.8.0               # Specific version requirement for py-moneyed
-django-money==1.1               # Django app for currency management
-certifi                         # Certifi is (most likely) installed through one of the requirements above
+django-allauth==0.45.0          # SSO for external providers via OpenID
+django-cleanup==5.1.0           # Manage deletion of old / unused uploaded files
+django-cors-headers==3.2.0      # CORS headers extension for DRF
+django-crispy-forms==1.11.2     # Form helpers
+django-debug-toolbar==2.2       # Debug / profiling toolbar
 django-error-report==0.2.0      # Error report viewer for the admin interface
-django-test-migrations==1.1.0   # Unit testing for database migrations 
+django-filter==2.4.0            # Extended filtering options
+django-formtools==2.3           # Form wizard tools
+django-import-export==2.5.0     # Data import / export for admin interface
+django-markdownify==0.8.0       # Markdown rendering
+django-markdownx==3.0.1         # Markdown form fields
+django-money==1.1               # Django app for currency management
+django-mptt==0.11.0             # Modified Preorder Tree Traversal
+django-q==1.3.4                 # Background task scheduling
+django-sql-utils==0.5.0         # Advanced query annotation / aggregation
+django-stdimage==5.1.1          # Advanced ImageField management
+django-test-migrations==1.1.0   # Unit testing for database migrations
+django-weasyprint==1.0.1        # django weasyprint integration
+djangorestframework==3.12.4     # DRF framework
+flake8==3.8.3                   # PEP checking
+gunicorn>=20.1.0                # Gunicorn web server
+inventree                       # Install the latest version of the InvenTree API python library
+pep8-naming==0.11.1             # PEP naming convention extension
+pillow==8.3.2                   # Image manipulation
+py-moneyed==0.8.0               # Specific version requirement for py-moneyed
+pygments==2.7.4                 # Syntax highlighting
 python-barcode[images]==0.13.1  # Barcode generator
 qrcode[pil]==6.1                # QR code generator
-django-q==1.3.4                 # Background task scheduling
-django-formtools==2.3           # Form wizard tools
-cryptography==3.4.8             # Cryptography support
-django-allauth==0.45.0          # SSO for external providers via OpenID
-
-inventree                       # Install the latest version of the InvenTree API python library
+rapidfuzz==0.7.6                # Fuzzy string matching
+tablib[xls,xlsx,yaml]           # Support for XLS and XLSX formats
+weasyprint==52.5                # PDF generation library (Note: in the future need to update to 53)