> Yeah - if I simply stick a rehash of 'ss' and 'structures' in that method it loads ok. I kinda doubt that is a truly principled solution though.

This should be fixed in trunk - that's an issue I signaled a long time 
ago. See point 8 in this post:

http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-August/221980.html

Stef