Japanese Type 4 75mm Anti-Aircraft Gun. The Type 4 75 mm AA gun was an anti-aircraft gun developed by the Imperial Japanese Army, which went into production in 1943. The Type 4 number was designated for the year the gun was accepted, 2604 in the Japanese imperial year calendar, or 1944 in the Gregorian calendar. Due to the lack of raw materials available and the great damage by air raids to its industrial infrastructure, only 70 units were made. These units were retained for defense of the Japanese home islands during World War II. In 1927, the Bofors company of Sweden had designed a new 75 mm AA gun which was purchased by the Royal Swedish Navy. This was developed into a mobile anti-aircraft gun with a wheeled carriage, which was exported to the Republic of China, Persia, and Thailand. Japanese forces captured examples of the Bofors M29 75mm AA gun from the National Revolutionary Army of the Kuomintang government of China, and the Army Technical Bureau was impressed with its per...