Colt marks their bolts with either "MP" or "MPC" according to my reference. The other manufacturers mark theirs with "MPF" (for an FN bolt), "MPD" (for a Diemeco bolt) etc. On my Colts the marks are no where near as "neat" as yours. Mine look like they were vibra etched by hand, and not the neat block letters in your pictures. The M stands for magnetic particle inspection, and the P for proof tested.