Civilian Marksmanship Program. It was a program designed to sell cheap obsolete military rifles, pistols, and ammunition to civilians to shoot target competitions with. It still exists today, though the supply of cheap surplus firearms has dwindled significantly. They never sell machine guns or assault rifles either.

The program arose at the end of WW2 because the US army discovered that the US is not a "Nation of Marksmen" but mostly a motley collection of "city slickers" that didn't know one end of a rifle from the other. Let alone actually shoot well enough to hit anything. It was taking too long to train recruits to use a firearm properly.

All you had to do was join a CMP approved target shooting club and shoot a set number of sanctioned matches to qualify to purchase a refurbished M1 Garand, M1 Carbine, 1903 Springfield, or 1911A1 pistol for very little money. It cost me $200US for a "Rack Grade" M1 Garand back in the day. And for the longest time there was no need for an FFL transfer either. The CMP would mail the firearm directly to your home.