The original C 96 was chambered in 7.63 Mauser which was not a German military caliber. Due to the slow production of standard 9 mm 08 pistols the German army had Mauser produce C 96 in 9 mm. As there was a large number of regular C 96 in use as officer pistols the 9 mm guns got the big red 9 on the grip to avoid confusion, especially as you could chamber a 9 mm round into a 7.63 chamber by mistake.
There is a large number of 9 mm C 96 around that were later drilled out from 7.63 (as you can't change the barrel on a C 96 when it's shot out). So value needs a lot of research to confirm true WWI origin.
F 135 - the right choice