It is what is called a "Waffenamt", or "Weapons Office", stamp. It signified that the weapon (or part) passed German Army inspection. Each inspector, or office, had a different number, and they moved around some, so the same model can have different numbers at different times. It's a whole field of study for German WWII gun collectors.
The Radom is a very well thought-of gun. There is at least one big book about them, and a couple of small ones. Gun Digest also did a big article about them, in the early 1990's I think.