Just to be extra anal, Diemaco is the only company to make a real C7 or C8 upper as they are the only ones to market them as such. With that out of the way, when Diemaco decided on the configuration of the rifle Colt wasn't yet making the A2, they were making the A1 with improvements such as;
-A2 sight base with .750 dia barrel (C7 only, the C8 uses a 14.5" pencil barrel and an A1 (.625) FSB
-A2 flash hider
-A2 lower
-case deflector
Anyway, if you want the absolutely best possible quality and as original as possible then buy one of the used Colt upper assemblies off of CMMG and replace the barrel. This would be accurate as some of the initial C7's and 8's contained some Colt parts. I'm Canadian military and I come across Colt manufactured bolts at work all the time.