High Quality Content by WIKIPEDIA articles! The Type 99 81 mm mortar was a Japanese mortar used primary by Imperial Japanese Army during World War II. The Type 99 designation was given to this gun as it was accepted in the year 2599 of the Japanese calendar (1939). The Type 99 81 mm mortar differs from the Type 97 81 mm infantry mortar in the shortness of its tube and in the method of firing. The differences between the Type 99 and the US 81-mm mortar, M1 are pronounced.