certainly looks like an FN or STG mockup for hand to hand combat training of some kind. No idea further than that.
what is an stg?
Those fricking swiss. Take a closer look at the stgw 57 on google and youtube, built like an armoured watch. Search Sig or SG 510 as well, same thing unless I'm missing something.
Also, they're chambered in 7.5x55, one heck of a nice cartridge.