Luger Stamping Question

[MAUSER88]

I have a question on this P-08. Pistol is all matching and a sear safety has been added.All matching numbers with a matching mag. with a number 1 and an Eagle L waffenampt. No grip strap markings. Bluing was refinished at some point post war but the straw seems original.

My question is on the left side of the frame are the numbers 1920.

I was always under the impression that these Police reworks were always stamped 1920 on the chamber next to the production date. Is this uncommon or common?