Bernd, great score. That's one of the nicest belts I've ever seen. The "O" on the back of the buckle are from the manufacturing as I believe they are injection molded.
One thing that always confuses me is how do you know what organization used a specific belt. Since brown belts were used by a few different organizations. Now Luftwaffe would use a blue gray backing but green seems more generic. Are they interchangeable? If a policeman walked into a store and needed a belt could he pick any belt (brown in this case) he wanted or was he limited? From looking in the Assmann catalog I can see several organizations used the same fittings. So is figuring out the proper organization all in the backing?