The MK IV Special Weapon was a more sophisticated follow-on to the MK III "Fat Man" implosion-type device detonated over Nagasaki on 9 August 1945 to force an end to World War II. It was the first production model and was stockpiled in service from 1949 to 1953. The MK IV had the same size and weight as the MK III and a very similar configuration, but its yield could be varied between 20 and 40 kilotons. The bomb could be safed in flight by opening the nose of the weapon and removing the capsule of fissionable material, then rearmed by replacing the capsule.