The USAAF basic scheme is summarized as follows:
(prefix)(type)-(chron. num.)(variant)-(production block)-(factory)
where "type" is a letter indicating basic category of aircraft (P for pursuit, B for bomber, C for transport, etc) and "chron. num" is thechronological number of the aircraft of that particular type. The "prefix" was not always used; it designated special features or roles (such as X for experimental). The "variant" was a letter in the sequence A, B, C,....which indicated the version of that particular aircraft in order of its entry into service.
The "production-block" number was introduced in 1942 to keep track of relatively minor modifications of aircraft not deemed to be sufficiently significant to merit a separate variant letter.
The "factory" code was an innovation also introduced at the beginning of World War 2 to keep track of the large numbers of aircraft manufacturers coming on line in support of the war effort. It was a two-letter code which indicated the plant where the aircraft was manufactured. Often, the same aircraft would be built by two or more different manufacturers.
For example, the first of the "bubble-canopy" Thunderbolts bore the designation P-47D-25-RE, which meant that it was the forty-seventh basic pursuit aircraft to be ordered by the Army, it was the fourth (D) basic variant, and was manufactured in the 25-th production block coming off the line at the Republic Aircraft Corporation in Farmingdale, New York. The production block number was significant since it represented an aircrafts modification state.
This designation scheme remained in force all throughout the Second World War. In 1948, the Army Air Forces were split off from the Army and became the Air Force. This evidently called for a new designation scheme. The four fighter categories were replaced by one, designated by F. However, it was decided NOT to start the chronological numbering system over again from one. Fighter aircraft already in service at the time of the change had the P replaced by an F, but kept their original chronological number. For example, the North American P-51 became the F-51, the Lockheed P-80 became the F-80, etc. As newer aircraft were ordered into service by the Air Force, they were assigned succeeding chronological numbers in the order in which they entered service.
The P-51D and P-51K were identical except that the former was fitted with a Hamilton Standard prop and the latter a Aeroproducts prop. The Aeroproducts suffered from vibration, a result of not being well balanced, and was disliked. Props were changed at times so the only way to differentiate the two models was to look at the data plate. All D and K models were fitted with the with Packard Merlin V-1650-7 engine.
The factory identified the aircraft by the order number (NA-XXX).
Inglewood, Ca plant (-NA)
Order NA-122, 1600 P-51D-20NA, 1600 P-51D-25NA, 800 P-51D-30NA
Order NA-110, first D models built were 100 P-51D-1NA, delivered unassembled to Australia. 80 of these aircraft were completed and designated CA-17 Mk.20. The CAC new built P-51s were designated CA-18. Versions would be the Mark 21, Mark 22, and Mark 23. The Merlin V-1650-7 was used in the CA-18 Mk.21 models. The CA-18 Mk.23 used the British built Rolls Royce Merlin 66 or 70 versions. A final batch to be designated CA-21 was cancelled. look at
ADF Serials - RAAF A68 CAC CA-17/CA-18 Mustang & North American P-51D/K Mustang you will note that only those aircraft supplied under the Lend/Lease scheme carry USAAF serials and the P-51 designation. In the engine department the British and American used different types of supercharger drive (Farman V epicyclic) and carburetor (SU v Bendix injection).
Dallas, Texas plant (-NT)
Order NA-111, 200 P-51D-5NT and 400 D-20NT.
Order NA-124, 600 P-51D-25NT and 400 P-51D-30NT.
Order NA-111, 200 P-51K-1NT, 400 P-51K-5NT, 600 P-51K-10NT, 200 P-51K-15NT, 100 P-51K-15NT
In British service
Order NA-109, the P-51D version was called the Mustang IV and the P-51K version was the Mustang IVa. The name “Mustang” was recommended to North American by the British.