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 a date and a serial number on the top of the receiver.
Never did find one with an identifiable mark to an armoury as most of them, so I understand, were assembled in small workshops by women.
I can see them being made in every satellite country of the Sovieticon Union due to the total simplicity of the arm.
The only problem with the PPSh 41 that was later rectified later in the war was the box magazine which was doubled in thickness for more strength.
You also needed a mag loader with the stick mag as they were an SOB to load without one past 10 to 12 rounds.