Al After WWI the german populace was offered a cash bounty for privately owned weapons, by the government. It was a decree called the "disarming of the people". The Government ordered that all of the Government Property weapons be stamped with the year of the decree, or "1920". Thus identifying them as Gov't property, to prevent anyone from collecting the bounty on weapons stolen from the Gov't.
This period from 1919 to 1933 was called the Weimar era. Your pistol is a "Weimar Police" pistol. Ed is a retired MP. A "Lousy Copper"

His handle is "Weimar Police" on another forum. He's really into them "Cop" guns.
Ron