 In the mid-1950s the Japan Ground Self-Defense Force contracted for one prototype each from Komatsu (SS1) and Mitsubishi Heavy Industries (SS2) powered by a 110 hp six-cylinder diesel engine and fitted with two recoilless rifles.They were delivered in 1956.A second series of prototypes was built with 4 recoilless rifles, but adoption of the American M40 recoilless rifle forced the reversion to two weapons.