I'm not sure I'd overthink it too much. I have this, possibly from as late as the 80s, and it has an FHF 905 movement in it rather than in-house.
With any luck you won't need access to any specific GP parts. For what it's worth, I've heard Simon Freese may have a GP parts account but you'd need to check that.
http://www.simonfreesewatchmakers.com/about-1/