It sounds like you have a transition model made up of obsolete old stock parts. Colt did this to some AR 15s in 1985 possibly in some 1986. It sound very much like a Sporter II R6500 in a transition version of the pre government model R6550. Is your barrel marked 1/ 7 upfront? It should be. No mag fence is normal on Colt Ar 15s till around 1989 give or take a year. You described a Government model barrel profile to a tee except with A1 sights instead of the normal A2 sights. Hence the transition model. The official Colt AR 15 Government model was not released till around 1987. I own a Sporter II 16" carbine I bought new in 1985 and it has a barrel marked 1/7, round forward assist, case deflector, no mag fence, and A1 sights also. Everything A2 except the A1 sights. I remember looking at a model just like yours when when I bought my CAR15 in 1985 at a dealer. Hope this helps ya.