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If you use some of the dirty Yugo ammo and with a rpm rate of anywhere between 800 to 1100 they all look like they are on fire at the ejection port.
I have photo's of one firing with 13 and 15...

Most WW 2 PPSh 41's that I examined had a mark just above the ejection port which was presumably a makers mark.
Also, a single mark that appeared to be a proof or inspectors mark beside it and, then...