type 89 grenade discharger

A portable grenade launcher employed by the Imperial Japanese Army in World War II. Due to its peculiar curved baseplate (which was designed to be braced against logs or large tree roots), American soldiers who captured them nicknamed them "knee mortars", assuming that the plate was meant to be held against a kneeling firer's leg above the grounded knee. In fact, the weapon's recoil is sufficiently stout that using it that way is a great way to suffer a broken femur.