Quote Originally Posted by Neal Myers View Post
Colt SP1 upper will have C M in the recess under the rear sight (right side).

Neal
Not necessarily. Those are forge marks and there could be a variety of companies making them. Mine says CH
Some earlier uppers had no marks at all.

To the OP.
The triangle over the square box at the front of the upper looks to be the colt verified proof over the c in box inspectors stamp, which would mean that upper left colt assembled as a rifle and test fired.
I vote it's an original A1 colt upper and not sp1 if both attach lug holes are the same size.

https://www.ar15.com/mobile/topic.html?b=3&f=123&t=401263