Ark Royal commissioned with a Mark III upper-deck mounting that did not penetrate the deck. Between-decks mountings extended below it and appeared on carriers and rebuilt capital ships such as Renown. The 4.5-inch gun also served on the Dido-class cruisers Scylla and Charybdis. A destroyer still needed to support a mounting able to fire at both high and low angles within a smaller ship's weight allowance. Having the gun in production solved only part of the problem.
Notable Point · 31:08-33:04
Why Ark Royal's 4.5-inch guns were hard to adopt for destroyers
Ark Royal had 4.5-inch guns in 1938. Destroyers still needed a mounting that worked within tighter weight limits.