OK, so maybe not super unusual, but definitely not run of the mill:
I have an upper receiver I bought from another guy that is black with engraved and painted T-markings. The barrel on it is a 16" dark gray parked M4 profile with chrome lining. The only markings on the barrel are the stamp "MP" on the top near the muzzle, which I take to mean "mag particle tsted" per standard barrel markings. The twist is not marked, but appears to be 1 in 9. the front sight base is forged with a bayonet lug, and has the raised forge markings "F01[symbols in a circle, maybe R1 or M]2" on the right side.
Any guessed who made it? The barrel nut doesn't have any signs of having been removed/reset or buggered, so I would hazard a guess it could be the original barrel on the upper receiver.
Oh, and it shoots great! Just want to know who made it!
Dawg