When the M14 rifle was developed to replace the M1 Garand, is was met with some uncertainty by the US Navy. The Navy had a lot of things to spend money on, and not a lot of need for a replacement for the M1 Garand (note that the Marine Corps did adopt the M14 despite being a component of the Navy). So instead of buying new rifles, the Navy opted to see if it could just convert its existing M1 rifles to the new 7.62x51mm cartridge.
Because of the similarity between the two cartridges, this seemed to be a fairly simple conversion. A chamber insert was designed which would fill the front of the chamber and allow the use of 7.62x51mm, with the resulting rifle designated the Mark 2 Mod 0. However, the chamber inserts tended to come loose with firing, so a modification was made. grooves were added to the front of the chamber to improve the adherence between insert and chamber. These also tended to come loose, and so the Navy was forced to resort to installing brand new barrels to make their conversions. This was more expensive than they would have liked, but was still much cheaper than buying new M14s, so they went ahead and bought 30,000 new barrels from Springfield Armory in 1965 and 1966. These were installed by H&R and American Machine & Foundry and became designated the Mk2 Mod1 rifles.
The only other modification necessary for the conversion was the addition of a white plastic spacer block in the magazine well. This simply blocked a shooter from inserting a clip of .30-06 cartridges. A .30-06 would not be able to chamber or fire in the new barrels, but the spacer block provided a handy reminder of the rifle’s new chambering. Because these conversions are quite simple, they are fairly easy guns to fake. This particular example includes sales paperwork from the CMP confirming its originality.