Hi Arno! Thanks a lot! To tell you the truth, I haven't looked between the lugs yet. Embarrassingly enough :) I'll check. No info on the detailed meaning of backcase markings yet.

There is one for sale at Lost Time anyway, the same was sold on MWR a short time ago. I did not know about the others.

I believe the case is unpolished, but according to the collector I bought it from, the last owner before him had it services at TAG Heuer in Switzerland. They might very well have changed the dial there, I have no idea. I'm trying to locate that owner now, he might be able to tell me more.

It's a great watch in its own right, and I hoping to gather a lot of info on its interesting history. I will surely write up an article if I unravel anything of interest.