The only factory complete 7.62x39 ARs that I am aware Colt ever manufactured are the R6830 and it's post-ban version, the MT6830. Both had A2 uppers and 16" barrels.
As you were advised, Colt also offered complete upper receiver assemblies that were given "R" prefixes. The R6850 is a 7.62x39 upper receiver assembly with a flat top receiver and a 16" barrel while the R6851 is the same but with a 20" barrel.
In the picture below, the rifle at the top is a factory original R6830, while the rifle on the bottom is a 7.62x39 marked lower with an M4 stock and a R6850 upper receiver assembly.