After the Armistice in November 1918, a lot of German soldiers simply went home, taking their weapons with them. Imperial Germany had strong gun laws, which carried over to the so-called Weimar Republic, but the laws were just ignored.
Finally, they passed a law banning the private ownership of military weapons, and requiring all issue arms to be turned in. When the guns were turned in, a bounty was paid and the guns were given the "1920" marking; it was the date of the law, but served as a state property mark. At the same time, all guns in the possession of the state were supposed to be marked the same way, but it seems doubtful that any more than a few actually were marked. That was to prevent military personnel from stealing government guns and turning them in for the bounty.
Luger pistols and Model 1898 Mausers are the guns most commonly seen with the 1920 marking, but other guns have it, indicating they were either turned in or were already in government depots. Since both Lugers and Mausers already were marked with the date of manufacture, the guns with the 1920 date are called "double date" by collectors.
The intent of the law was never really fulfilled, and it did not prevent the Nazis and other groups from obtaining weapons; sympathizers in the Army and the police kept the SA and other right wing groups well supplied with guns because they were seen as a counter to the Communists.
Jim
