Why wartime anti-aircraft fits varied ship by ship

How can researchers determine the 20 mm and 40 mm anti-aircraft fit a ship carried as the war progressed?

Use ship-specific evidence, because wartime fits changed informally and could include locally acquired or captured weapons not reflected in the original class specification.

Small anti-aircraft weapons changed too quickly and informally for one class-level table to remain authoritative. Ships began with documented fits, but crews and dockyards added weapons as equipment became available and threats changed. Some Royal Navy cruisers and destroyers even carried captured Italian Army 40 mm guns acquired near naval facilities. The resulting arrangement could be individual to a particular ship and date rather than shared across a class. Researchers therefore need refit records, photographs, and ship-specific evidence instead of relying only on the original design armament.