I was told it was a 6520 but now I've heard it may not be.  Its a upper I bought at a pawn shop with a new Doublestar lower I just got from Lone Star wholesale.  The barrel has an O or a C space then MP 5.56 Nato 1/7.  Its a little heavier under the handguards then the end of the barrel but not much.  I guess I'd like to know what I really have.  The only mark on the bolt is  D,  also stamped upside down under the handguard on the barrel is a 9/97

The barrel pic is poor,  but a  "C MP 5.56 NATO 1/7" would indicate a COLT®  barrel..

I have a factory assembled COLT rifle from 1989, it has the same  "C MP" marks on the A2 upper, so it should be Genuine COLT®..

Can't do you anything for the bolt carrier assembly.

better pics of the bbl. markings would help but so far it looks like a homebuilt with a few colt parts.

Email or IM me if you want to sell or trade the carrier for a Colt.