[4] On 29 March 1922, a new designation system was introduced with a reorganization of U.S. naval aviation under the Bureau of Aeronautics.
[5] The system conveyed its information in the form: For example, F4U-1A referred to a minor modification (A) to the first major subtype (1) of Chance-Vought's (U) fourth (4) fighter (F) design.
For instance, the letter "A" was used both for deletion of the tailhook from an aircraft normally so equipped (e.g. the Douglas SBD-5A, used from land bases) and for addition of this equipment to a land-based aircraft; for amphibious versions of flying boats (e.g. the Consolidated PBY-5A); for armament added to a normally unarmed type; and for miscellaneous modifications (e.g. the aforementioned F4U-1A).
[6] A significant wartime exception to this system was existing United States Army Air Forces (USAAF) types adopted by the Navy, such as the North American B-25 Mitchell; in some such cases, the minor modification letter simply mirrored the USAAF sub-type letter, e.g. the B-25H became the PBJ-1H.
For many planes, the mission letters and design numbers were retained, as the Douglas AD Skyraider became the A-1 and the McDonnell F4H Phantom II became the F-4.
A very similar system, the short system, was adopted by the Imperial Japanese Navy Air Service in the late 1920s that differed only in the use of the 1 for the first assigned type, having letters assigned to match Japanese aircraft and manufacturers, and not having a different number series for each manufacturer, meaning that the A6M was the sixth model of single engine fighter (A), adopted into Imperial Navy service, which was built by Mitsubishi (M), as opposed to the US system in which it would have meant the sixth fighter model specifically from Mitsubishi.