I need information on a SP1 type upper that came on a preban EA lower. The upper has a 16 inch barrel, bayonet leg, with a military style 3 prong suppressor. The upper has forward assist but no case deflector. Under the A1 sight adjustment wheel has P M in raised letters. Does anyone know who manufactured the upper.

                 Bluedevil        

That certainly doesn't sound like a factory configuration to me. I'm not sure about the P, although the M usually means the upper was forged by Martin Marietta. All SP1 carbine uppers were large-hole pivot, slick side with no forward assist. They also came with an A1 birdcage flash suppressor.

This is pure speculation, but P M is not a Colt marking.  It might, however, be a govt contractor marking on a unit that had the front pin hole opened up for the large screw. Incidentally, I was unaware that anyone but Colt ever made a large screw lower. Learn something every day, I guess. [:D]

As it happens, I just acquired a forward assist Colt upper reciever with a large SP1 type front hole.  The letters under the rear sight are C  H and the Colt proof marks are stamped just ahead of the ejection port.  The forward assist is not the teardrop type, but the early large button with the flat milled on the inside. Since the flat was milled in to clear the increased dimension at the rear of the A2 type lower, I guess the thing was originally one of the early commercial A2 types.  I know that Colt spent several years selling ARs that still had A1 sights, after the adoption of the A2, but I didn't realize that they were still using the large pin fronts at that point.

It has a standard size pivot hole. The  16 inch barrel is larger in diameter under the m4 type handgards and smaller in diameter up to the threads where the suppressor is fastened. The bolt carrier and bolt are make by Colt.

No, there are no markings of any kind on the barrel.

        Bluedevil [devil]