I have seen a few ads for these (the 6521) but only as complete rifles. They have all been non-HBAR barrels, A2 style upper (unless the seller swapped them already), and had 1/7 twist rate barrels.
If yours is flat top, 1/9, and HBAR, I would say it is a 6721 despite what your label states. I am not very heavy into Colt AR history, but the 6721 is the only 5.56 1/9 twist I have seen on any of their rifles.
That is not to say you don't have what the label states. It is just odd that they wouldn't change the model number. Come to think of it, weren't flat tops pretty rare, if not practically non-existent in 1999-2000? At least on the civilian market? I remember carbines becoming more prolific around that time, but they still had A2 style upper receivers, if I recall correctly.