They all appear to be stamped "MPC".
Doesn't that make them Colt?
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OK, here is a pic of a proported FN bolt with the semi-elliptical depressions behind the lugs. I care not the authenticity of the bolts' manufacture but I am curious to learn the name and function of these bolt characteristics.
(Mod: If pic is not in public domain, please delete it.)
http://i201.photobucket.com/albums/a.../Bolt-FN-1.jpg
I have seen Colt bolts with other types of MP markings. I believe some were MPJ and MPY. I would say that some of those are possibly FN and maybe LMT.
Here's my FN bolts, marked MPF - I believe they are legit.
They have the same semi-elliptical cuts.
Feel free to save this image and use it whenever you like. It's the best I could do with a cell phone (I spend my money on guns, not cameras :D)
http://img594.imageshack.us/img594/1140/53473589.jpg
Here's a shot of the F marked extractors:
http://img827.imageshack.us/img827/3000/fn2u.jpg
Colt bolts marked MP_ or MP__ up to four place stamps.
FN bolts have been MPF with an underscore under one of the letters, usually the F
I have no clue as to the name of the area in question. As far as purpose I would guess that area was trim down to allow more free movement of the bolt inside the carrier causing less jams due to excessive carbon build up.