Reichswehr property stamp: German law of August 1920 required all Germans to hand into the government all military arms, which included arms stolen after WW1 had ended. The mark chosen to identify Reichswehr weapons was the “1920". If the weapon were found in the wrong hands, then it could be identified as stolen and the person in possession of the weapon could be prosecuted. The mark was used into the 1930's. The marking can often be crude and unprofessional because, typically, the pistols were taken to the local police station to be marked. Some appear to be neatly marked from a stamping die containing all 4 digits together, and some can be crudely misaligned, with each digit having been stamped individually. I don't know if the law specified a location for the stamp.