Also, in "US & Allied Submarine Successes in the Pacific and Far East during WW II", latest edition 2009 by John D. Alden and Craig R. McDonald, I find:

KOSEI M., 16.12.1942, 233 tons, merchant cargo ship, attack SW of Kushiro, unknown extent of damage, prob. erroneous attribution.
K.M., 13.3.1943, merchant cargo ship, 3262 grt, hit claimed by SUNFISH, or ran aground to avoid torpedo, sank in storm 14.3.
K.M., 7.4.1943, 8282 or 8237 grt, naval auxiliary refrigerated cargo ship, torpedoed by USS TUNNY. Sank 9.4. off Truk
K.M., 11.11.1943, auxiliary cargo ship, 3551 grt, sunk by USS SARGO.
K.M., 27.1.1944, aux. cargo, 2205 grt, sunk by THRESHER S of Formosa.
K.M., 13.10.1944, merchant coaster, 79 or 99 grt, sunk by HMS STURDY W of Celebes.
KOSEI MARU # 2, 4.7.1945, merchant motor-sailer, 44 grt, sunk by USS TIRANTE off Shantung Peninsula.

And that makes seven KOSEI MARUs..