That's a really lovely watch. The DSM and field effect displays aren't interchangable so it's a third generation stem set lcd module in there (it's probably very similar to the one in the Kuatron lcd that Azimuth posted in the for sale section a while back with excellent pictures).
From what I can gather from reading up on it and my own collection the Optel modules went.
1) Stem set DSM. One function, round module (BWC, Delvina etc)
2) Stem set DSM, One function, rectangular module (Optcom 1, Walchron etc)
3) Stem set LCD, both round and rectangular module (Segtronic etc)
4) Back set LCD, round module (Sandoz etc)
5) Several generations of plastic based modules with pusher command and setting buttons (digitaire, spectra etc)
The first three are the most collectable so it'll make good money should you decide to sell it. Where did you find it? - I'll be gutted if it slipped under my radar on ebay