Looking for info regarding the meaning of a letter 'S' stamped on the right side of my IBM receiver. It is stamped directly beneath the stock line to the far left. It is the size of a standard typed capitol letter. There are no other non standard markings. Serial number is 3,659,574.
The only variations I am aware of are the standard "I.B.M. CORP" on the rear of the receiver and the same with "AO" (for Auto-Ordinance) stamped below the serial number on the bevel of the receiver. Any other marks on the receiver are probably just inspector's marks or production tracking marks whose meaning is lost to time. I've seen similar marks, usually letters, on other carbine receivers as well.