Japanese submarines armed with 15.24cm/6” guns according to the U.S. Joint Intelligence Committee. Daily summary dated 16 April 1942

An item reported that Japanese submarines seemed to be armed with 15.24xm/6”guns regarded the unexploded example found back on board of a British merchant ship recently attacked by a Japanese submarine in the Indian Ocean.

Source

Map Room Papers (Roosevelt Administration), 1942 - 1945. U.S. Joint Intelligence Committee. Daily summary No. 127 dated 16 April 1942.