No idea of fire forming your own but I would forget it. PPU is not that much and is good brass. The base is unique, I had to mill out a Bot head to get mine to work (larger, its off no other cartridge unlike the 7.92/30-06 that is parent to a lot of cases)
Lapua makes 7.5 cases but costly and hard to impossible to fine.
RCBS makes a good die for it. Don't get caught up in the K-31 vs 1911, the RCBS works for both just fine. Not an issue for you, I fire formed cases from PPU and segregated them so I have 3 different groups (Savage 7.5 barrel, K-31 and the 1911)
GP-11 are Berden primed and essentially unusable sans a lot of unneeded mucking with.
They are required for import, used to be under the barrel. Nice ones were laser etched.
Latter ugly ones scratched into the top receiver. Yours likely came in before that was required. Mine either did or was not enforced. I had a Modl of 1917 that had it under the barrel (SWANVT).
If you look at an imported AK, its on the left side of the receiver. Not pretty but they are not pretty guns and ok.