Hello...
The Waffenamt or WAa marking was a little eagle over a number, on these Mausers, usually found on the web between the trigger guard and the frontstrap, in the corner. Most common marking on these, when a WAa denotes that the pistol was entered into German military service. If it is an Army pistol, it would have a 135 WAa mark on the left hand side. The WAa marking denotes Army usage. Without a WAa mark, the gun was commercial, probably pressed into service anyway, a private purchase by an officer or such.
Cheers,
Gary