The E61 anthrax bomblet was an American biological sub-munition for the E133 cluster bomb. The Flettner rotor bomblet was a U.S. biological sub-munition that was never mass-produced. The original weaponized form used the M114 bursting bomblet in M33 cluster bombs. Each spherical bomblet held of sarin nerve agent. Each bomblet would then explode using its own detonator. The M83 fragmentation bomblet was used in the US M28 and M29 cluster bombs. The spherical bomblet was the biological version of the Sarin-filled M139 chemical bomblet. The M139 was a spherical bomblet that was filled with of sarin (GB). These warheads incorporated the M125 bomblet and the M114 bomblet. The first was the CBU-58 which uses the BLU-63 bomblet. The experimental Flettner rotor bomblet was also developed during this time period. The E120 was a spherical bomblet with a diameter of 11.4 centimeters. The Dutch vehicles do feature a bomblet protection package. By 1964 the successor M139 bomblet was ready for production. Much like the M139 bomblet, the E120 had exterior "vanes".