The Battleship Callisto (renamed Admiral McCaw) had its five twin 15” put into storage while the hull became an aircraft carrier. But what to do with the twin 15”? A third Mars class battleship was ordered but with a completely new layout to take the four twin 15” in place of the three triple 15”. It was indistinguishable in looks from the Mars and Mercury except for the gun layout. Completed in 1939 the new ship had new radar and electronic systems that were not fitted to the others till later in 1939 and early 1940.
