Based on my unprofessional opinion, the upper you have is a Colt upper.
I believe the one you are talking about is a 20" A2 with no bayonet lug but has an A2 flash hider. Correct?
I heard that these uppers were put on some 7.62x39 lowers that Colt had to get rid of them and that's why they are marked .223 on the side.
Again, I don't claim to be in the know...but it came from a good source. Sounds good anyway