I have a match to that gun marked with the same Nowotny-Prague markings. It is in a 7.8 x 36 R cartridge. It is considered a Tesching cartridge, which is similar to the English rook cartridges. I ended up making cases from 30-30 brass that worked well. It was a real challenge to make brass for, but is a lot of fun to shoot and does well. I suspect yours might be a similar cartridge, but with smaller bore. There were a lot of similar cartridges to that size.

Bruce