Whats with the '9' on the grips on some artillerys?

[Sergio Natali]

Walter PPK is a smaller version of the Walter PP.
It's quite a cute little gun, widely used by German Police forces during WWII
Adolf Hitler shot himself with his PPK 7.65mm in his Führerbunkern in 1945.
I've seen some really nice ones recently, and not that expensive either, for instance the one enclosed Zella Mehlis costs about USD 1K
Somehow I've never managed to be interested in them (so far), I tend to prefer bigger handguns like 1911, Lugers and SIGs

Sergio