Just don't even ask how long I worked on this tonight for a new mod I'm working on, and I'm not even 100% sure I'll like the mod once I'm done!
Anyway, the relbits tuning appears to use the hash of the class name (RelationshipBit) for the resource id, and not the hash of the instance type (relbit). So when extracting that resource (at least with my XML Extractor) it ends up getting the resource id number wrong.
Once I changed that in testing my mod, persistence works fine with a 32 bit instance id, but using a 64 bit value will cause a ValueError exception when trying to save the game.
So yeah, long story short you need to use resource id 0x0904DF10 instead of 0x074DFB83 and use a 32 bit instance id and Bob's your uncle - or maybe Scumbumbo's your uncle, whatever.
I guess I'll have to write a special case into XML Extractor to handle that so it gets numbered properly in the future. But not today, I'm tired.
ETA - Uploaded v3.5.4 of XML Extractor to MTS with a fix to number that resource type properly.