I was wondering about this as well. The Shikishima would have been the first in her class to be outfitted with the 510mm 20 inch main guns. Unfortunately for japan the ship was not completed in time for the war but in 1941 the 2 hulls were converted from a battleship to an aircraft carrier creating the Shinano and the Kure.
I was wondering if there is a possibility of the Japanese receiving the 20 inch double barrel turrets that was planned for the next generation design of the yamato class battleships. Shikishima was going to be the first of its kind designated the A-150 class of battleship for the Japanese.