The MRF stands for Mongolian Rim Fire. It was a special caliber revolver manufactured in honor of Mao Tse Tung, commemorating the 70th million victim of the people's democratic revolution.
Ammo is very scarce, and can only be purchased in three places: China, the DNC, or the White House.
Actually MRF stands for Magnum Rim Fire, so it is simply a 22 mag. The 651 is an excellent revolver and would make an excellent addition to your collection.