Grandfathers 1917 Artillery Luger

[CavScoutEurope]

Is it his SSN or his serial number that he was given while in the service? I know that my grandfather had both memorized. and he would often put his old serial number on things to ID that they were his. My dad did the same thing when he was in the Army, and I also did it when I was in the Army. everything that I did this to was never stolen by my fellow soldiers.