I have a Bushmaster, and the bolt and carrier have a dark coating that I can't identify. It's not Parkerizing or blueing. Also, the carrier has a small area on the left side where something was ground off, but the coating was applied afterward. I bought the upper new and it's only been fired a few times. I've often wondered what could have been stamped on the carrier that had to be ground off before plating, or whatever it is.

Maybe it's that mil spec phosphate finish!