The crux of the matter is that the “1920” stamp is not a date, but rather a property mark. When the Treaty of Versailles was being enforced, it restricted the German military to just 100,000 men , and strictly limited the number and types of arms they were allowed to have. At the same time, strict civilian gun control was enforced in an attempt to remove the leftover military arms from the war from Germany. In total, nearly 5 million firearms were collected and destroyed between roughly 1919 and 1921.
The 1920 stamp on guns indicated formal (and legal) German military ownership of a particular weapon. This made it accountable to the Inter-Allied Military Control Commission (to the extent that they tabulated such things) and also made the gun easily identifiable if it were stolen from a military unit.