Okay, I have I believe an original full Colt M16 B/BC assy.  It may have come with either an M4 upper, or Commando upper (both Colt).  I checked the Bolt & Bolt Carrier FAQ at biggerhammer.net.

Bushmaster should be painted MPN.  Colt should stamped MPC.  This bolt is stamped (or however they do it, it almost lookes like engraving) in the same manner as Colt but with MPNC.  MP means Magnetic Particle tested, and C means Colt, but what the heck is MPNC???  Carrier is also stamped C and is for F/A.  What is the N for?  Anyone???

OK, lotta questions here. The first two letters are inspection marks showing a GO in testing, the last mark is the makers mark. I say "last mark" instead of "third mark" bc there has been a change to military markings.

Quick review:
M=[b]M[/b]agnetic particle tested
P=[b]P[/b]roof load tested
C=[b]C[/b]olt.
N=[b]N[/b]ational Aerospace

The MPNC bolts are marked using the new marking system that includes a date code (in this case it's an "N") in the coding. The bolt is a Colt one made in military date code year N. No, I don't know what year that is.

Thanks Tweak,

I just wanted to be sure I didn't get ripped off with a bolt switcheroo.  I guess this means it a recent Colt is all.  Cool.  That year "N" thing is reminding me of my MP5 mags and their letter date coding...hmmm...

I'm not sure when the date coding took effect but it was taught at the Colt Armorers class last year. I think it may have come about as a result of the accelerated breakage of the M4 bolts.

Hey, [pink]celebrate[/pink]![party]
You got a "mil[rolleyes]spec" bolt. Hehe.
[beer]

The others I've seen are made from the incorrect, and of course cheaper, steel.