If it's an SP1 Colt, the upper, bolt and carrier are probaably original , since nobody but Colt ever made that big hole front screw type AR. The barrel is drfinitely NOT original. Colt barrels of that era are marked C MP and either nothing after the MP (very early), C after the MP (chrome chamber), B after the MP (Early chrome bore)m or "chrome bore" (late fully chrome bore).
Somebody put a GI surplus replacement barrel on that piece at some point. I'm assuming that the marking under the rear sight says "C" and one or two other letters on it.
Re the carrier, yes, once the forward assist was adopted for military use, Colt just went ahead and used the same pieces, with the notches, to build the AR15 carriers. There should be a "C" on the left side of the carrier. The bolt head markings are also Colt.