I have this 3-cell, pebble grain black leather pouch and can't recall how I acquired it or what it for originally. It looks like it would fit Suomi Coffins.
It looks like a post war, Warsaw Pact holster for angular magazines; possibly handgun magazines. I can't gauge the size very well, but it looks relatively small.
Looks like it would accept 3 magazines of roughly 8 to 8 1/2" length by 1 1/2" deep by 1 3/8" wide. I think the dimensions are too large for a handgun.
The rivets and tarnishing copper clasp make me think Soviet. The quality of execution and materials makes me think East German. An image search for pebbled leather east german brings this p38 holster up, which is pretty similar.