I ran some bolts this weekend at the guns show.   The vendor says they are Colt, but I don't know enough to tell the difference.  One of the bolts is marked MP w/ one leg of the M & one of the P running together.  The other is marked MPKC.  Any way of finding out if they are Colt, or if not Colt, who made them?

The MP marked piece is probably Colt; but if it is, it's VERY old.  Back when Colt first brought out the AR15/M16 pieces in the 1960s "MP" was the standard marking. And yes, the M and P were run together.  When contracts were given  to H&R and GM for M16s, during the Viet Nam War, Colt sdded the "C" to ID pieces rthey made.

The other piece is definitely Colt; and is fairly recent in origin. The "C"  IDs it.  The K is a date code.  Date codes are a fairly recent addition to the markings . Can't recall exactly when they started w/them. I think about 12-15 years ago.

LMT bolts have m p marks but they don't run together.  BCM bolts have m p marks and from I can tell from the picture at bravocompany they do run together.

Some other markings are MPIC & MPEC in addition to just C. Dunno what the eras are for those particular markings, however.

The MP marked bolt has the MP running lengthwise on the bolt, as opposed to around the circumference of the bolt like the other style linked in this thread.  Assuming it's a Colt bolt of that vintage, is there any reason I shouldn't use it as planned, or does it have a lot of extra value to a collector of Colts?

Quoted: The MP marked bolt has the MP running lengthwise on the bolt, as opposed to around the circumference of the bolt like the other style linked in this thread.  Assuming it's a Colt bolt of that vintage, is there any reason I shouldn't use it as planned, or does it have a lot of extra value to a collector of Colts?

Can't speak for anyone else, but even though I'm a retro nut, the MP only bolt woiuldn't mean anything added to me.

Bought a bunch of Milsurp bolts a few years back. One was an MP; the others all MP C. They all headspaced within useable limits; though the MP did close on the No Go, but not the Field guage. I'd definitely check the headspace on any rifle you install it in.  

Quoted: LMT bolts have m p marks but they don't run together.  BCM bolts have m p marks and from I can tell from the picture at bravocompany they do run together.

the BCM MP looks laser etched... not rollmarked...

the newer colts look like this: