Bolt marked MPXC. Any ideas what this means?

MP = Magnetic Particle inspected
X = monthly coding Colt used (your bolt would have been made in July of 1997)
C = Colt

From the book "Black Rifle II", page 299:

                       Colt's Month and Year Date Coding System
From September, 1995 until November, 2001, Colt utilized a monthly coding system, whereby an additional letter was inserted between the "P" and the "C" to indicate the month & year of manufacture. The system begins with the letter "A", so the markings on a bolt made in September, 1995 read "MPAC, in characters 0.090" high.

ETA: The letter "C" was not used in the monthly coding system, in order to avoid confusion.

Quoted: MP = Magnetic Particle inspected X = monthly coding Colt used (your bolt would have been made in July of 1997) C = Colt From the book "Black Rifle II", page 299:                        Colt's Month and Year Date Coding System From September, 1995 until November, 2001, Colt utilized a monthly coding system, whereby an additional letter was inserted between the "P" and the "C" to indicate the month & year of manufacture. The system begins with the letter "A", so the markings on a bolt made in September, 1995 read "MPAC, in characters 0.009" high. ETA: The letter "C" was not used in the monthly coding system, in order to avoid confusion.

I think you mean 0.09" not 0.009" high letters.  Also, when they ran out of letters, they repeated the sequence with underlined third letters, and when those ran out, they continued with overlined third letters, always skipping the letter "C". Guess they gave up on the idea after that.

Quoted: Quoted: MP = Magnetic Particle inspected X = monthly coding Colt used (your bolt would have been made in July of 1997) C = Colt From the book "Black Rifle II", page 299:                        Colt's Month and Year Date Coding System From September, 1995 until November, 2001, Colt utilized a monthly coding system, whereby an additional letter was inserted between the "P" and the "C" to indicate the month & year of manufacture. The system begins with the letter "A", so the markings on a bolt made in September, 1995 read "MPAC, in characters 0.009" high. ETA: The letter "C" was not used in the monthly coding system, in order to avoid confusion. I think you mean 0.09" not 0.009" high letters.  Also, when they ran out of letters, they repeated the sequence with underlined third letters, and when those ran out, they continued with overlined third letters, always skipping the letter "C". Guess they gave up on the idea after that.

Yeah that is what I meant, but not what I typed. Thanks for pointing out my error.

I left the rest of the details out cause they did not pertain to the OPs bolt, & I referenced the page number in the book, which I copied the text from.

Wow, that's awesome. I thought that it might have been magnetic particle something Colt, but wasn't sure. This bolt group came in a like new Oly PCR carbine I bought in about 2000-2001. The carrier is the M16 style and has perfect key staking.

I bought the rifle (A2 carbine) with carry handle scope mount, scope, Harris bipod, 6 30 round mags, and case for $600. It was my first AR and it hasn't been out of the safe in a while. I got it out today to clean and lube and noticed the markings.