[1] In 1938, the design was reduced in length to 16.5 meters (54 ft) with 20 tons displacement and a second 57 mm tank gun added; 11 boats of this configuration were built by Osaka Iron Works (大阪鉄工所, Ōsaka Tekkosho).

[1] In 1939, the displacement of the ship was reduced to 16.5 tons and the second 57 mm tank gun removed; 19 boats of this configuration were built by Osaka Iron Works.

[1] After the Battle of the Bismarck Sea (2–4 March 1943) where eight transports and four destroyers were lost to enemy air attacks, the Japanese increasingly relied on convoys of barges escorted by armored boats to replenish or evacuate their forces.

[5] Due to her slow speed, she was eventually outclassed by the American patrol torpedo boat and a Japanese equivalent was developed (ja:カロ艇).

She is not to be confused with the ST-class armored boats built by Mitsubishi Heavy Industries Ltd. at their Yokohama Shipyard for riverine patrol in China.