diff --git a/stix2/base.py b/stix2/base.py
index 7336285..910042a 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -537,8 +537,10 @@ _JSON_ESCAPE_MAP = {
     "f": "\f",
     "n": "\n",
     "r": "\r",
-    "t": "\t"
+    "t": "\t",
 }
+
+
 def _un_json_escape(json_string):
     """
     Removes JSON string literal escapes.  We should undo these things Python's
diff --git a/stix2/test/v21/test_deterministic_ids.py b/stix2/test/v21/test_deterministic_ids.py
index da72142..1e6e2d4 100644
--- a/stix2/test/v21/test_deterministic_ids.py
+++ b/stix2/test/v21/test_deterministic_ids.py
@@ -189,14 +189,16 @@ def test_empty_hash():
         SomeSCO(hashes={})
 
 
-@pytest.mark.parametrize("json_escaped, expected_unescaped", [
-    ("", ""),
-    ("a", "a"),
-    (r"\n", "\n"),
-    (r"\n\r\b\t\\\/\"", "\n\r\b\t\\/\""),
-    (r"\\n", r"\n"),
-    (r"\\\n", "\\\n")
-])
+@pytest.mark.parametrize(
+    "json_escaped, expected_unescaped", [
+        ("", ""),
+        ("a", "a"),
+        (r"\n", "\n"),
+        (r"\n\r\b\t\\\/\"", "\n\r\b\t\\/\""),
+        (r"\\n", r"\n"),
+        (r"\\\n", "\\\n"),
+    ],
+)
 def test_json_unescaping(json_escaped, expected_unescaped):
     actual_unescaped = stix2.base._un_json_escape(json_escaped)
     assert actual_unescaped == expected_unescaped
diff --git a/stix2/test/v21/test_observed_data.py b/stix2/test/v21/test_observed_data.py
index ceca8f1..e0fa456 100644
--- a/stix2/test/v21/test_observed_data.py
+++ b/stix2/test/v21/test_observed_data.py
@@ -1,6 +1,5 @@
 import datetime as dt
 import re
-import uuid
 
 import pytest
 import pytz
@@ -900,6 +899,27 @@ def test_file_example_with_RasterImageExt_Object():
     assert f.extensions["raster-image-ext"].exif_tags["XResolution"] == 4928
 
 
+def test_file_with_archive_ext_object():
+    ad = stix2.v21.Directory(path="archived/path")
+    f_obj = stix2.v21.File(
+        name="foo", extensions={
+            "archive-ext": {
+                "contains_refs": [ad, ],
+            },
+        },
+    )
+    f_ref = stix2.v21.File(
+        name="foo", extensions={
+            "archive-ext": {
+                "contains_refs": [ad.id, ],
+            },
+        },
+    )
+
+    assert f_obj["id"] == f_ref["id"]
+    assert f_obj["extensions"]["archive-ext"]["contains_refs"][0] == ad["id"]
+
+
 RASTER_IMAGE_EXT = """{
 "type": "observed-data",
 "spec_version": "2.1",
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index d73c1cf..a15d5bd 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -14,8 +14,7 @@ from ..properties import (
     BinaryProperty, BooleanProperty, DictionaryProperty,
     EmbeddedObjectProperty, EnumProperty, ExtensionsProperty, FloatProperty,
     HashesProperty, HexProperty, IDProperty, IntegerProperty, ListProperty,
-    ObjectReferenceProperty, ReferenceProperty, StringProperty,
-    TimestampProperty, TypeProperty,
+    ReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
 )
 from .base import _Extension, _Observable, _STIXBase21
 from .common import GranularMarking
@@ -144,7 +143,7 @@ class EmailMIMEComponent(_STIXBase21):
 
     _properties = OrderedDict([
         ('body', StringProperty()),
-        ('body_raw_ref', ObjectReferenceProperty(valid_types=['artifact', 'file'])),
+        ('body_raw_ref', ReferenceProperty(valid_types=['artifact', 'file'], spec_version="2.1")),
         ('content_type', StringProperty()),
         ('content_disposition', StringProperty()),
     ])
@@ -201,7 +200,7 @@ class ArchiveExt(_Extension):
 
     _type = 'archive-ext'
     _properties = OrderedDict([
-        ('contains_refs', ListProperty(ObjectReferenceProperty(valid_types=['file', 'directory']), required=True)),
+        ('contains_refs', ListProperty(ReferenceProperty(valid_types=['file', 'directory'], spec_version="2.1"), required=True)),
         ('comment', StringProperty()),
     ])
 
@@ -465,7 +464,7 @@ class HTTPRequestExt(_Extension):
         ('request_version', StringProperty()),
         ('request_header', DictionaryProperty(spec_version='2.1')),
         ('message_body_length', IntegerProperty()),
-        ('message_body_data_ref', ObjectReferenceProperty(valid_types='artifact')),
+        ('message_body_data_ref', ReferenceProperty(valid_types='artifact', spec_version="2.1")),
     ])
 
 
@@ -654,7 +653,7 @@ class WindowsServiceExt(_Extension):
                 "SERVICE_SYSTEM_ALERT",
             ]),
         ),
-        ('service_dll_refs', ListProperty(ObjectReferenceProperty(valid_types='file'))),
+        ('service_dll_refs', ListProperty(ReferenceProperty(valid_types='file', spec_version="2.1"))),
         (
             'service_type', EnumProperty(allowed=[
                 "SERVICE_KERNEL_DRIVER",