Funny thing, in my area, the surplus 7.65 Arg. is or was fairly common, used to be every couple of weeks I'd wipe out the local seller, $6 bucks a box. Seemed like it was always there and I just kept getting 10-15 boxes, what ever was there. I've missed a couple of times getting it recently, but now last time I went in its, 18bucks a box.
Off the top of my head, I think even a FnFal or maybe the Cetme was made in 7.65 Arg. I know there were other heavy machine guns in the caliber, maybe like the M1919 or Vickers or something.