Original C 96 Mauser in 9mm Parabellum.
Branded with a “nine” filled in red to distinguish from more common 7.63mm.
Many original 7.63s have been ruined in recent years by folks firing 7.62x25 Tokerev in them.
That ammo is much too hot for these old guns, it batters them to pieces in short order.
I have a lot of stuff of this vintage and fire them less than I used to.
Value is way up and breaking a small numbered part is very detracting to that.
1917 "Artillery Luger I have owned 40 years. Fun to shoot with stock, but getting too valuable to risk.
I have a few mismatched pieces I fire more often.