Left side: stamped near the rear, low, 2781, w/EP upside-down-h 305, tilted number 4 near the base.
Back: upside-down 7, another partial character which I can't quite make out, but looks sort of like an M.
Front: Low, near base, partial backwards letter k in box.
The above is the only original mag I have, and it is not the same serial number as my rifle (4931).
The other two are after-market
1) Santa Fe - Japan stamped on back. No numbers. Works well.
2) No name, but has stamped in large letters on the side something like "SVT-40, 7.62X54R, automatic rifle". I have it out at work modifying it so that it will work properly in the rifle. I'll post a correction here tomorrow.