Not to contradict Arno's point but discussion on these over on the MWF (Military Watch Forum) tends to point towards all issued watches having the 1550SG OR the rarer 1551SGSZ model numbers on the case

There were also a smaller number of civilian watches sold. This could be a mis-match of civillian case with military caseback.

Its also possible a replacement case has been used by Sinn who took over the service & supply of spare parts at a later stage. They offered a service from minor movement service through to full restoration with replacement service hands, dials & cases being commonly swapped out to create a very new looking watch.

To the original poster
I understand your concern about your watch. It looks lovely in its condition but there is a very real concern the watch may not have been issued. Can i suggest you take this discussion over to the MWF forum & ask their members ( who are very knowledgable on issued watches ).
Good luck - Darren