Quoted:
Yes, it is a nice, Original Vietnam issue XM16E1 or early M16A1 upper.  Your bolt carrier is also Colt. They were not marked with a "C" until 1967 when GM and H&R started making weapons.  You can tell that it is a Colt carrier by the large hole on the right side of the carrier where the FP retainer comes through.  Aftermarket, and GM and H&R carriers have a smaller bevelled hole, among other differences.  You can use this complete upper to build a mid to late XM16E1 or an early M16A1.

Quoted:
In case you were interested in a second opinion, I would also agree that its a late XM16E1 or early M16A1.  Its authentic Colt though because of the proof markings in front of the ejection port and the markings on the barrel.  There's really no way to tell which rollmark it would have had on the lower because Colt used so many variations of parts on these transitional rifles, but the MP C barrel marking says that the rifle dates from about Oct. 1967 to 1969.  It was the first barrel with chrome chamber, but does not have a chrome bore.  Nice piece you have there.