Budster:
According to Black Rifle II----
-the very first 18,000 barrels made were made by Winchester for Colt. NO markings at all and 1/14" twist but still 5.56 chamber. After that it was solely Colt made barrels until the other contractors began producing M16's.
-Summer of 1963- Colt was still the only AR producer. Colt switched the barrels (5.56) to 1/12 and so had a "12" on the bottom of the bl. Still 5.56. But soon after started marking the barrels also with "CC" for chrome chamber and "CB" for chrome bore. So they would have "12" and some had CC and/or CB. There was an "M" under the front sight to indicate the MP testing and a small triangle Colt inspection mark as well. At some point- the "M" moved to the rest of the info on the front of the barrel and the triangle went away.
-1967- The chrome plating became standard. Also other companies had begun producing barrels (all 5.56mm) and so you see what for example the OP has:
Colt Magnetic P article (tested) Chrome Bore and the other barrel --same thing but the second "C" indicating chrome chamber.
For example a "SAK" produced barrel might read---SAK MP Chrome Bore.
-But all these barrels had a 5.56 Chamber as that was the standard
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