The rectangular mark that sits 1/3rd the way across the base is from the flat ejector that whacks them out of the gun. I remember in the early 80's, we had a load of 'foreign' blank that was in boxes marked with a green/white/green band. One of the ammo techs told me that it was Nigerian from some as-per-usual one sided trade deal where we gave them, say, 1,000 as new FTR's Bedford RL trucks, worth millions and they gave us £50,000 worth of pure horse manure blank ammo from some bankrupt crooked ammo plant, owned by the big Bwana's son. But I digress as is usual...........
Anyway, this stuff was full of dynamite mixed with ammatol mixed with PE4 and a bit of semtex for good measure. Even on the lowest gas setting, it was shaking the poor Bren guns apart and the cases flew out, hit the ground with such force, they bounced up and flew into orbit around the sun. Anyway, they took it all back and replaced it with some RG stuff after a few months, when the need for rear echelon TA troops in training was well past... as is usual!