WWII US MkII Fragmentation Grenade. Green Painted High Explosive Pineapple Grenade with yellow band & M6 Series Fuze Head. Note Many Yellow Grenades Were Over Painted With Green to conceal in the field of battle. Body is marked E. The Mk 2 grenade replaced the failed Mk 1 grenade used by the U.S. military during World War I. 44 million were ordered and more than 21 million were completed (it was possible to convert Mk 1 grenade bodies to the Mk 2 configuration) before the war ended, but few reached American troops overseas. It was formally standardized in 1920. The Mk 2, like the Mk 1, was manufactured of cast iron with a grooved surface divided into 40 knobs in 5 rows of 8 columns. This was to enhance fragmentation (in practice, it was found that the grooves did not enhance fragmentation as much as desired) and provide a better grip when handling and throwing the grenade. The grooves and knobs gave it the appearance of a pineapple, and are the origin of that nickname. It was also commonly referred to as a "frag" grenade, in contrast to other types of grenades such as the Mk 3 grenade concussion grenade. The Mk 2 used the M5, M6, M10, M11, or M204 series fuses. The early M5, and the later M6 and M204 series detonating fuses, were used on high explosive-filled grenades. The M10 and M11 series igniting fuses were used on low explosive-filled ones. The early fuses had many problems. In the M5, moisture could get in under the foil fuse cap, causing the weapon to fail to detonate. The early fuses were not completely silent and made a loud "bang" and produced sparks when activated. They also made a faint "hissing" sound while burning, potentially alerting the enemy of their presence. The M10, used during the interwar period, and the M10A1, used early in WWII, sometimes prematurely detonated when the flash from the primer hit the explosive charge rather than the delay fuse. They were replaced by the M10A2 and M10A3. A less common type of igniting fuse was the M11. The M6A4C had a delay of 4 seconds. The M5 and M11, like the M10, M10A1 and M10A2, had a delay of 4 to 5 seconds. The later M10A3 had a delay of 4.5 to 5.3 seconds. In 1944, the M6A4C was replaced by the silent and more reliable 4 to 5 second delay M204 or M204A1 fuse. Due to the large number of grenades already issued, few grenades with the new fuses were used in combat during WWII.