I think this one was based on the French 37mle SA18. Let's see what Wikipedia has to say. here it is. Derived from the 1916 modele. en.wikipedia.org/wiki/Puteaux_SA_18 Japan had industrialized in record time. This left her at a loss for modern weapon designs after the first world war. France provided Japan with two very important weapons: 1) The ultramodern Hotchkiss 25mm Antitank gun and 2) the 81mm Brandt mortar. A lot of water over the dam due to those specific weapons. During the interregnum, the Japanese became proponents of the 25mm caliber light Dual Purpose cannon, and of the mortar; light mortars, medium mortars, heavy mortars . . . rocket mortars. Adherence to the former crippled the IJN and perhaps decisively defeated Japanese aims in the Pacific "Drive South". Adherence to the latter concept led to the development of the 50mm "Knee Mortar" so much a part of squad tactics, and of the archetypal 70mm Type 93 Daitaiho (Battalion Gun), a most interesting weapon. The SA18 was another French weapon widely adopted at the time (1920s), though it was well on the way to obsolescence by the time the USA cut off lead additives and gasoline exports to Japan in 1941. It retained relevance in the invasion of China and the Civil war that followed. In effect, it's a high power grenade launcher. I don't think an AP round was even considered for it until the French began to adapt it for arming its new Chars Légères in the 1930s. fr.wikipedia.org/wiki/Char_l%C3%A9ger Though the US Army adopted it in WW1, the "French 37" as it is also called was largely superseded by the advent of the Rifle Launched Grenade.
