The shape is all wrong for the C96 as the magazine would not fit into the holster properly. Pic of a C96 holster attached. It is much more squared off to take the gun properly. These also came as a wooden holster which doubled as a detachable shoulder stock.
My guess as to the subject holster would be for a Gasser Montenegren revolver which is a very big beast. Barrel length of these is 235mm with an overall length of 375mm which would fit with what you have. They weighed 1.3kg.
https://en.wikipedia.org/wiki/Gasser_M1870
I assume that there is no sign of any marks on the leather?
Stu