All three marks of assembled guns are interchangeable units. There is very little difference, from the operating point of view, between the Mark 2 (British) and the Marks 2 and 4 (U.S.N.). There are certain differences in the Marks 2 and 4 guns that affect interchangeability of parts; when individual parts are not interchangeable between Marks 2 and 4, the assembly or subassembly containing those parts is usually interchangeable.
For the ammunition for Guns Marks 2 and 4, refer to OP 911, Chapter 9.
The Mark 14 Gun Sight was first adapted to the 20-mm and 1.1" gun about the middle of 1942 as it became available. These were the primary air-defense weapons systems at the time.