OK, so I'm thinking about where gestalt ID could be coming from. Apparently it's not in the ROM since the ROM is identical yet the IDs are different. The ROM must set the gestalt ID based on what hardware it finds on the logic board. So somewhere outside the ROM, on the logic board, there is an actual electronic difference between the TAM and 6500 logic boards.

I guess I always thought, assumed even, that the gestalt ID was stored directly in ROM, but this proves that it indeed is in RAM at least for this model. This would explain how a utility like Wish I Were could exist; all it has to do is modify this RAM location.

What would be really cool is to be able to run a debugger and step through just until the gestalt ID is set, then modify it to be the TAM gestalt and see what happens if you resume operation. I bet you'd hear the TAM startup sound on a 6500.