I found a number of variations Once i looked. Some were marked M-16A1, and many have been marked M16A1. Some had SERIAL basically stamped on there before the serial quantity, and Other people didn't. It's the early reciever without any fence round the mag launch and all early style release https://gold-cup-trophy-colt-4590000.madmouseblog.com/11719840/not-known-facts-about-colt-combat-unit-rail-gun-45