I didn't check the Bigger Hammer website before starting this, so you may have already gotten all the info you needed. If not, I can tell you that it's a Colt early A2 series upper (C=Colt)
The MB indicates the forging people Colt used on this piece. My own Colt A2 upper is only marked C M. (Gee, maybe yours is better than mine. You've got an extra letter

) I say early because of your description of the color. Anyway it's a good unit. Hope you get much use from it.