The very first had discolored subdials and a solid sna-on case back. Then the dial changed to „normal“ colored sub-dials but the snap-on case back remained. 3971 was also offered with snap-on glas case back.
All the above are quite rare and trade at a premium.
All othe 2nd series, 3 series, 4 series are quite similar in terms of market appreciation(with differences based on case material, of course).
Of course, the black stick dial (with P or G case) only came with 3rd/4th series. These black dial versions are as rare as the esrly snap-on models… But simetimes trade a bit lower still.