The '1920' marking was the property mark of the new post WW1 German Weimar Republic. The Versailles Treaty allowed them only a limited number of small arms for internal security, police, ect.
They were designated to be marked '1920' as a way of keeping track of the number allowed by the treaty.
Most of the Luger pistols were simply battlefield recycled arms that underwent a rebuilding process and others were new, near new arms that were simply stamped with the additional yr/date.
A '1920' marked pistol is one that was property of the German Post WW1 govt (Weimar Gov't)
It could have been issued to anyone from town, city, German States police depts, border security, railway police, postal police, ect, ect.
Some are marked as such as well as the holsters.