Colts Firearms Division
Thought they started stamping them in 1978. Before it was Colts Patent Firearms Mfg Co. I have seen in AR-15 civilian ones with starting serial number range SP83500. That (4975721) couldn't be made in 1972? I have seen a M16-A1 in serial this high as this 5662000 in this same stamping as yours. So, I think they a made a few limited number made after 1973 and on until 1978? I believe yours is made around 1978 or so? There's no way to know for sure.??