Looks like a Błyskawica.
https://en.wikipedia.org/wiki/B%C5%8...submachine_gun
There is a thread called "Living my heritage - The Błyskawica" over on WeaponsGuild with a lot of info.
Leszek Erenfeicht did an excellent write up on these waaay back in the forgotten days (pre-youtube) of Forgotten Weapons.
https://www.forgottenweapons.com/sub...lyskawica-smg/
I wonder if a BRP tube could be parts-swapped into being a Błyskawica. I imagine parts kits are hard to come by, since those production numbers are crazy low.