Quote History Quoted:
Upper receiver is not Colt. The T-marks and m4 stamp above gas tube hole are not typical of Colt factory uppers... Bolt/bcg are probably not colt as well.
View Quote
My 6920 has T-marks and an M4 stamp above the gas tube, however the T-marks are a different font and the M4 stamp on mine is smaller. Additionally mine has a C stamped about 1/2" behind the keyhole forge mark. There are no marks on the carrier, the bolt has (had) a white circle on the bottom that has since worn off.
I don't think these markings can be used to definitively identify Colt as the manufacturer as they seem to have changed more than once over the years.
They aren't worth more than any other mil spec parts just because they are Colt so don't pay a premium. Additionally I don't think it's really possible for the seller to prove 100% that every part came from Colt so your options are to take them at their word or buy new from a reputable dealer.