diff --git a/stix2/custom.py b/stix2/custom.py
index 5912dbf..0d83742 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -4,8 +4,8 @@ import six
 
 from .base import _cls_init
 from .registration import (
-    _register_marking, _register_object, _register_observable,
-    _register_observable_extension,
+    _get_extension_class, _register_extension, _register_marking,
+    _register_object, _register_observable,
 )
 
 
diff --git a/stix2/parsing.py b/stix2/parsing.py
index ea6ffd1..001f523 100644
--- a/stix2/parsing.py
+++ b/stix2/parsing.py
@@ -1,9 +1,6 @@
 """STIX2 Core parsing methods."""
 
 import copy
-import importlib
-import pkgutil
-import re
 
 from . import registry
 from .exceptions import ParseError
diff --git a/stix2/registration.py b/stix2/registration.py
index 4ec019a..28d43ba 100644
--- a/stix2/registration.py
+++ b/stix2/registration.py
@@ -1,14 +1,13 @@
 import re
 
-from . import registry
-from .base import _DomainObject, _Observable
+from . import registry, version
+from .base import _DomainObject
 from .exceptions import DuplicateRegistrationError
 from .properties import _validate_type
 from .utils import PREFIX_21_REGEX, get_class_hierarchy_names
-from .version import DEFAULT_VERSION
 
 
-def _register_object(new_type, version=DEFAULT_VERSION):
+def _register_object(new_type, version=version.DEFAULT_VERSION):
     """Register a custom STIX Object type.
 
     Args:
@@ -32,7 +31,7 @@ def _register_object(new_type, version=DEFAULT_VERSION):
     properties = new_type._properties
 
     if not version:
-        version = DEFAULT_VERSION
+        version = version.DEFAULT_VERSION
 
     if version == "2.1":
         for prop_name, prop in properties.items():
@@ -45,7 +44,7 @@ def _register_object(new_type, version=DEFAULT_VERSION):
     OBJ_MAP[new_type._type] = new_type
 
 
-def _register_marking(new_marking, version=DEFAULT_VERSION):
+def _register_marking(new_marking, version=version.DEFAULT_VERSION):
     """Register a custom STIX Marking Definition type.
 
     Args:
@@ -59,7 +58,7 @@ def _register_marking(new_marking, version=DEFAULT_VERSION):
     properties = new_marking._properties
 
     if not version:
-        version = DEFAULT_VERSION
+        version = version.DEFAULT_VERSION
 
     _validate_type(mark_type, version)
 
@@ -74,7 +73,7 @@ def _register_marking(new_marking, version=DEFAULT_VERSION):
     OBJ_MAP_MARKING[mark_type] = new_marking
 
 
-def _register_observable(new_observable, version=DEFAULT_VERSION):
+def _register_observable(new_observable, version=version.DEFAULT_VERSION):
     """Register a custom STIX Cyber Observable type.
 
     Args:
@@ -86,7 +85,7 @@ def _register_observable(new_observable, version=DEFAULT_VERSION):
     properties = new_observable._properties
 
     if not version:
-        version = DEFAULT_VERSION
+        version = version.DEFAULT_VERSION
 
     if version == "2.0":
         # If using STIX2.0, check properties ending in "_ref/s" are ObjectReferenceProperties
@@ -133,27 +132,25 @@ def _register_observable(new_observable, version=DEFAULT_VERSION):
     OBJ_MAP_OBSERVABLE[new_observable._type] = new_observable
 
 
-def _register_observable_extension(
-    observable, new_extension, version=DEFAULT_VERSION,
+def _get_extension_class(extension_uuid, version):
+    """Retrieve a registered class Extension"""
+    return registry.STIX2_OBJ_MAPS[version]['extensions'].get(extension_uuid)
+
+
+def _register_extension(
+    new_extension, version=version.DEFAULT_VERSION,
 ):
-    """Register a custom extension to a STIX Cyber Observable type.
+    """Register a custom extension to any STIX Object type.
 
     Args:
-        observable: An observable class or instance
-        new_extension (class): A class to register in the Observables
-            Extensions map.
+        new_extension (class): A class to register in the Extensions map.
         version (str): Which STIX2 version to use. (e.g. "2.0", "2.1").
             Defaults to the latest supported version.
 
     """
-    obs_class = observable if isinstance(observable, type) else \
-        type(observable)
     ext_type = new_extension._type
     properties = new_extension._properties
 
-    if not issubclass(obs_class, _Observable):
-        raise ValueError("'observable' must be a valid Observable class!")
-
     _validate_type(ext_type, version)
 
     if not new_extension._properties:
@@ -163,37 +160,18 @@ def _register_observable_extension(
         )
 
     if version == "2.1":
-        if not ext_type.endswith('-ext'):
+        if not (ext_type.endswith('-ext') or ext_type.startswith('extension-definition--')):
             raise ValueError(
-                "Invalid extension type name '%s': must end with '-ext'." %
+                "Invalid extension type name '%s': must end with '-ext' or start with 'extension-definition--<UUID>'." %
                 ext_type,
             )
 
-        for prop_name, prop_value in properties.items():
+        for prop_name in properties.keys():
             if not re.match(PREFIX_21_REGEX, prop_name):
                 raise ValueError("Property name '%s' must begin with an alpha character." % prop_name)
 
-    try:
-        observable_type = observable._type
-    except AttributeError:
-        raise ValueError(
-            "Unknown observable type. Custom observables must be "
-            "created with the @CustomObservable decorator.",
-        )
+    EXT_MAP = registry.STIX2_OBJ_MAPS[version]['extensions']
 
-    OBJ_MAP_OBSERVABLE = registry.STIX2_OBJ_MAPS[version]['observables']
-    EXT_MAP = registry.STIX2_OBJ_MAPS[version]['observable-extensions']
-
-    try:
-        if ext_type in EXT_MAP[observable_type].keys():
-            raise DuplicateRegistrationError("Observable Extension", ext_type)
-        EXT_MAP[observable_type][ext_type] = new_extension
-    except KeyError:
-        if observable_type not in OBJ_MAP_OBSERVABLE:
-            raise ValueError(
-                "Unknown observable type '%s'. Custom observables "
-                "must be created with the @CustomObservable decorator."
-                % observable_type,
-            )
-        else:
-            EXT_MAP[observable_type] = {ext_type: new_extension}
+    if ext_type in EXT_MAP:
+        raise DuplicateRegistrationError("Extension", ext_type)
+    EXT_MAP[ext_type] = new_extension
diff --git a/stix2/registry.py b/stix2/registry.py
index 3dcc3a5..90e2826 100644
--- a/stix2/registry.py
+++ b/stix2/registry.py
@@ -37,7 +37,7 @@ def _collect_stix2_mappings():
                 STIX2_OBJ_MAPS[ver] = {}
                 STIX2_OBJ_MAPS[ver]['objects'] = mod.OBJ_MAP
                 STIX2_OBJ_MAPS[ver]['observables'] = mod.OBJ_MAP_OBSERVABLE
-                STIX2_OBJ_MAPS[ver]['observable-extensions'] = mod.EXT_MAP
+                STIX2_OBJ_MAPS[ver]['extensions'] = mod.EXT_MAP
             elif re.match(r'^stix2\.v2[0-9]\.common$', name) and is_pkg is False:
                 ver = _stix_vid_to_version(stix_vid)
                 mod = importlib.import_module(name, str(top_level_module.__name__))
diff --git a/stix2/v21/bundle.py b/stix2/v21/bundle.py
index 5497da5..990dfc1 100644
--- a/stix2/v21/bundle.py
+++ b/stix2/v21/bundle.py
@@ -40,7 +40,7 @@ class Bundle(_STIXBase21):
     def get_obj(self, obj_uuid):
         if "objects" in self._inner:
             found_objs = [elem for elem in self.objects if elem['id'] == obj_uuid]
-            if found_objs == []:
+            if not found_objs:
                 raise KeyError("'%s' does not match the id property of any of the bundle's objects" % obj_uuid)
             return found_objs
         else: