I think 500/500+ trapdoor expansions are pin compatible, but 500 expansions won't work in a 500+ (and vice versa) because of some memory addressing differences. At least, not to their advertised specs.
As for the different models of 500s available, I've got a really strange one: An OCS A500 with a case badge saying "A-500."
I guess the best way to solve the mystery behind this machine is to pop the lid, see if there's anything attached inside.
Something else I just thought of, I think there were some A500 RAM expansions that connected to the trapdoor and Gary, but still had passthrough connectors to attach something else (RTC? One of those PowerPC emulators?). Make sure you're dealing with the genuine trapdoor, and not an extender board.
