The last series of letters/numbers are a cage code. I do not know if they are used in the civilian world, but I have used them for years in the Air Force when researching parts. That particular cage code "1M291" belongs to "Check-Mate Industries Inc" of New York (www address:
www.checkmateindustries.com/index.asp). The only other info I found mentioned they may be Vietnam era mags. Perhaps the company can provide better info.