You likely know more than I do, but wasn't Mauser ammo actually 7.63 in label?

Given the 7.62 and the apparent Eastern European language, I'd lean towards that being 7.62 Tokarev, and maybe for submachineguns. If so, it's supposed to be loaded too hot for the Broomhandle (whereas the other way around was okay for the Tok-designed guns).