I know pics are best, but I just "found" it, and haven't had time to get out the camera.

Starting from the front, it has an A1 FH, lock washer, and a "C"-marked FSB. The 20" barrel is marked "C MP C", but no twist markings. I haven't taken off the triangle handguards to see if there are any markings near the chamber. It has a flat slip ring.

The upper has a teardrop FA, raised sight markings, no forge codes, and what I think may be a VP mark forward of the small rectangle port cover. The front pivot pin hole is standard GI size. It's gray, really clean, and almost looks unused.

After reading the stickies here and on RBR I think it could be a 603, but I've been wrong (frequently) before. Please chime in.

Even though it won't be a totally correct "retro" if I use an A2 lower, I think this upper may end up on my PSA Vietnam lower with the custom engraved MACV patch on the magwell.

Early M16A1 upper with chrome chamber. 1968-1971

The barrel is a 1/12 twist and should be used with 55 grain .223/5.56, the barrel markings mean Colt Magnetic and Particle Inspected Chrome Lined.

Thanks, but I know what the markings mean, and that the early ones were either 1/14 or 1/12. I'm just trying to determine the series, 602, 603, 604, etc. and year(s) of production.

As iSmith said, early M16A1, which would be a 603.