The only markings I can find on the upper are "C MP CHROME BORE" on the barrel and a "C" on the right side of the front sight base.  I can't find any markings on the upper receiver other than an arrow followed by an "R" above and ahead of the windage knob.  Slab side, small front pivot hole.  Should the upper receiver have any other marking?  

1) If it's an SP1, it has a LARGE pivot pin hole.

2) There should be a "C" and another letter (could be any of several) in the recess under the rear sight.  It it concievable that, if it's a very early piece, before Colt started using more than one forge, that those letters aren't there, but then the barrel markings would be different, since the "chome bore" markings would indicate a later barrel.

Finally, look at the right side of the upper, just ahead and above the ejection port.  There should be two very tiny and light stampings at that point; a triangle and a square. Each will have something inside of the figure.  Those are the ptoof marks.  You may have to use a strong light and magnifying glass to see them, because they're usually very lightly stamped.

If the proof marks are there, it's a Colt unit. If they're not, it's somebody elses.

Lot of guys will try and pass off small hole USAF surplus M16A1 uppers (without forward assist) as SP1 uppers.  Buyer Beware!

"C" inside a square
triangle barely visible - looks like "<"

Quoted: Lot of guys will try and pass off small hole USAF surplus M16A1 uppers (without forward assist) as SP1 uppers.  Buyer Beware!

Well, actually, they're M16(no A1on 'em) uppers, but if he has an SP1 lower, it would have to be attached with an adapter screw. And why would somone pull the original smoothside upper and replace it with another smoothside upper?  Don't know about where y'all are, but around here you can pick up takeoff SP1 uppers for $50-$75 , from guys who have A2d or M4d their SP1s.  The Air Force surp pieces go for $20-$30 more, precisely because they're small hole.

gregw45, that picture was great.  I DO have those markings on the upper, the "C"was just very faint, not nearly as clear as yours, but definitely there as was the triangle.  I thought at first they were just tool marks or something from when the barrel was installed.  So how do you explain the small front pivot hole?  Were there no SP-1s made with small front pivot holes, "limited-edition" maybe?

Quoted: So how do you explain the small front pivot hole?  Were there no SP-1s made with small front pivot holes, "limited-edition" maybe?

Nope. Like shamayim posted above, it's most likely an original M16 upper (small front pivot hole). It's the original "slick side" - SP1s followed and had the large front pivot pin hole.

M16 upper