debugging ZEXP import - no module named - plone

Question

I am attempting to zexp import a site from what should have been the same Plone version and addon packages. However, when I do so I get
Module ZPublisher.Publish, line 60, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 46, in call_object
Module OFS.ObjectManager, line 620, in manage_importObject
Module OFS.ObjectManager, line 638, in _importObjectFromFile
Module ZODB.ExportImport, line 92, in importFile
Module transaction._transaction, line 260, in savepoint
Module transaction._transaction, line 257, in savepoint
Module transaction._transaction, line 690, in __init__
Module ZODB.Connection, line 1123, in savepoint
Module ZODB.Connection, line 587, in _commit
Module ZODB.ExportImport, line 176, in _importDuringCommit
ImportError: No module named MyOldPackage.Content
This is an old Archetypes class that has since been migrated to a replacement in Dexterity. I thought I had removed all references but apparently something got missed. I get that I have to remove all objects that use this class but there's the rub - how do I find which objects use them?
I thought I had found something in the CMFEditions tools but that might be a red herring. Trying to export portal_historiesstorage yields the same error so I thought perhaps some old versions of past content were stuck there. Deleted shadow storage and rebuilt it, so it's now blank (ZMI page confirms this). But I'm still getting the error when trying to import just this tool - how is it still trying to reference this class? This is why I think it might be a red herring and it's inheriting something from the portal object, but I'm having a rough time trying to find where this class is referenced.
edit:
It looks like I have something bad in zvc_repo - I'm not sure the difference in what shadow storage and this are supposed to do. Here's a proposed script. Can anyone with better CMFEditions or ZopeVersionControl comment on how this sounds?
tool = portal.portal_historiesstorage
from ZODB.broken import PersistentBroken
bad_repo_ids = set()
for sequence in tool.zvc_repo._histories:
for version in tool.zvc_repo[sequence]._versions:
obj = tool.zvc_repo[sequence].getVersionById(version)._data._object.object
if isinstance(obj, PersistentBroken):
bad_repo_ids.add(sequence)
for bid in bad_repo_ids:
del tool.zvc_repo._histories[bid]