diff --git a/docs/source/contributing/pep.rst b/docs/source/contributing/pep.rst
index d763a96e2..80dcecde5 100644
--- a/docs/source/contributing/pep.rst
+++ b/docs/source/contributing/pep.rst
@@ -741,18 +741,20 @@ is worth knowing before benchmarking against them: ``pyshark``, ``pypcap`` and
 Checksum and Integrity Verification
 -----------------------------------
 
-Eight protocols parse a checksum or CRC field —
+Ten protocols parse a checksum or CRC field —
 :class:`~pcapkit.protocols.internet.hip.HIP`,
 :class:`~pcapkit.protocols.internet.hopopt.HOPOPT`,
 :class:`~pcapkit.protocols.internet.ipv4.IPv4`,
 :class:`~pcapkit.protocols.internet.ipv6_opts.IPv6_Opts`,
+:class:`~pcapkit.protocols.internet.ipx.IPX`,
+:class:`~pcapkit.protocols.internet.mh.MH`,
 :class:`~pcapkit.protocols.link.ospf.OSPF`,
 :class:`~pcapkit.protocols.transport.sctp.SCTP`,
 :class:`~pcapkit.protocols.transport.tcp.TCP` and
 :class:`~pcapkit.protocols.transport.udp.UDP` — and exactly one of them checks
 whether the value is *right*:
 :attr:`SCTP.checksum_valid <pcapkit.protocols.transport.sctp.SCTP.checksum_valid>`.
-For the other seven the field is recorded and never questioned, so a corrupted
+For the other nine the field is recorded and never questioned, so a corrupted
 capture parses as cleanly as an intact one.
 
 **These are two different problems and they want separating**, because only one
diff --git a/pcapkit/corekit/sentinels.py b/pcapkit/corekit/sentinels.py
index bf8788740..3606da6da 100644
--- a/pcapkit/corekit/sentinels.py
+++ b/pcapkit/corekit/sentinels.py
@@ -363,8 +363,8 @@ class NoDefaultType:
     compares by value rather than identity, and reload staleness is a
     *tracked* defect class here for other constructs -- see
     :meth:`pcapkit.protocols.protocol.ProtocolBase._lookup_next_layer`'s own
-    docstring note citing GitHub issues :issue:`425` and :issue:`555`, and
-    :mod:`tests.protocols.test_dispatch_default_resolution_unit`'s own
+    docstring note citing GitHub issues :issue:`421`, :issue:`425` and
+    :issue:`555`, and :mod:`tests.protocols.test_dispatch_default_resolution_unit`'s own
     ``test_no_stale_class_survives_a_module_reload``, which reloads a module
     deliberately to pin the fix for exactly that class of bug elsewhere. A
     *future* comparison site written the vulnerable way -- a bare ``is
diff --git a/pcapkit/protocols/protocol.py b/pcapkit/protocols/protocol.py
index ebae0bd33..1f9d0766b 100644
--- a/pcapkit/protocols/protocol.py
+++ b/pcapkit/protocols/protocol.py
@@ -1741,8 +1741,10 @@ def _lookup_next_layer(registry: 'DefaultDict[int, ModuleDescriptor[ProtocolBase
             :func:`importlib.import_module` -- see :issue:`574`. Memoising the resolved
             class here instead, whether under ``proto``, in ``registry``'s
             default factory, or in a cache beside the registry, would retain a
-            class that :func:`importlib.reload` then makes stale; :issue:`425` at
-            this layer and :issue:`555` at the schema layer are all that same defect.
+            class that :func:`importlib.reload` then makes stale; :issue:`421` at
+            this layer, :issue:`425` for the option, chunk and block registries
+            beside it, and :issue:`555` at the schema layer are all that same
+            defect.
 
         """
         protocol = ProtocolBase._lookup_registry(registry, proto)
diff --git a/tests/protocols/test_dispatch_default_resolution_unit.py b/tests/protocols/test_dispatch_default_resolution_unit.py
index 1c6c56205..9f5d169ca 100644
--- a/tests/protocols/test_dispatch_default_resolution_unit.py
+++ b/tests/protocols/test_dispatch_default_resolution_unit.py
@@ -5,10 +5,11 @@
 produces -- normally :class:`~pcapkit.protocols.misc.raw.Raw`. That resolution is
 deliberately **not** written back into the registry, because the registry is a
 class-level :class:`collections.defaultdict` and recording a miss in it is the
-defect GitHub issue #425 reported and pull request #428 fixed at this layer, and #560 fixed at the schema
-layer. The cost of not writing it back is that every unrecognised frame resolves
-the same descriptor again: 48 of the 52 resolutions an extraction of
-:file:`many_interfaces.pcapng` performs.
+defect GitHub issue #421 reported and pull request #426 fixed at this layer,
+pull request #428 extended to the option, chunk and block registries, and pull
+request #560 fixed at the schema layer. The cost of not writing it back is that
+every unrecognised frame resolves the same descriptor again: 48 of the 52
+resolutions an extraction of :file:`many_interfaces.pcapng` performs.
 
 Proposed by @Ts-Boom in GitHub pull request #563, which paid that cost with a
 class-level cache of resolved classes and no invalidation -- so a