Please scroll down to see all the images. This is a DWM 1916 Treaty of Versailles gun also stamped 1920.  I believe this gun was reworked twice.  Once during the Weimar period and again during the early Third Reich era.  The proof to the left of the normal Imperial proofs appears to be HZa.JL.16 which was only used in 1928, according to World Of Lugers by Costanzo.  The stamp to the left of that appears to be HZa.JL.2.  I assume that was stamped when the S/42 replacement barrel was installed..