I believe they changed/updated their "coding" system in 1965, where 3 then referred to solid white gold.
They seem to have applied the same system/s across their entire range.
A rough summary from a polerouter perspective (e.g. no polerouters were in chrome/nickel plate so it hasnt been included):
https://universalgenevepolerouter.com/decoding-polerouter-reference-numbers/
You should be able to extend these coding systems to the entire model range.
Click to expand...