The Mk 1 grenade is a fragmentation hand grenade used by American forces during World War I. According to its designers, it was to be the "simplest", yet most "fool-proof", grenade ever made. However,
The Mk 2 grenade is a fragmentation-type anti-personnel hand grenade introduced by the United States Armed Forces in 1918.
Mk 1 grenade
…major problems appeared when the grenade was used in the field. It was retired from service before the war ended, replaced in 1918 with the improved Mk 2 grenade used through World War II. The Mk 1 is a time-fused fragmentation grenade and has 32 serrations on it. To start the fuse, the user has to pull the…
World War II–era Mk 2 grenade
Instructional document showing correct throwing technique (1918): "Correct position of thumb when throwing […] is important for otherwise firing lever will not be thrown over when grenade leaves the hand."