I think the caseback is not original to the watch. If I am not mistaken, the D1 version of the 142 has a special dial with a logo in place of the 9 o'clock continuous seconds register.
I have a friend who owns a similar two-register 140/142. His contains a Lemania 5100 just as yours. The dial script is the same, too. The medallion (with serial number) on your movement is typical of an early version of the 5100, so my guess is that it was produced in the early 1980's. The serial number is low for a 5100, confirming my suspicion that it is from early in the 140/142 production range.
I have seen a few of these and believe your particular version is a transitional model between the Lemania 1341-powered 140 and the three-subdial 142. That is only my theory, but suffice to say that you should keep it! The other versions of this watch I have seen use a plain caseback (no engravings) with straight-grain finish.
-Daniel