Prior to October 1962, there were six Skyhawk designations: A4D-l, A4D-2, A4D-2N, A4D-3 (none built), A4D-4 (none built), and A4D-5. The letter "A" stood for the aircraft mission: Attack. The "4D" indicated it was the fourth attack aircraft manufactured by the Douglas Aircraft Company (later McDonnell Douglas, now Boeing). The number following the dash indicated the numerical sequence within that model, the A4D-2N, for example, being a night/all-weather version of the A4D-2.Early in the A4D's history, Douglas Aircraft referred to any export version of the A4D as the "DA101". This designation saw little useage.Legend has it that the current military aircraft designation system took root one day in 1962 when then Secretary of Defense Robert McNamara was confused during a discussion of two Navy fighter aircraft -- the Douglas F4D Skyray and the McDonnell F4H Phantom. Mr. McNamara apparently was surprised to discover that the two F4s were different aircraft. Consequently, Mr. McNamara decreed that a standard designation system be established and, since October 1962, all military aircraft have been identified by the new (and current) system: The initial letter designates the aircraft mission -- "A" for Attack, "F" for Fighter, "T" for Trainer, etc.; the dash number represents the numerical sequence within that mission (A-l, A-3, A-4, A-6, A-7, etc.); and the final letter represents an alphabetical sub-sequence within the numerical sequence (A-4A, A-4B, A-4C, etc.).When the 1962 change hit the Navy inventory, the letter designating the manufacturer was dropped. The A4D-1 became the A-4A, the A4D-2 became A-4B, the A4D-2N became A-4C, and the A4D-5 became A-4E. The A-4D designation was skipped to avoid confusion with the earlier A4D. The A4D-3 and A4D-4 reached the design stage but were never produced; A4D-6 lost the VAL (light attack) competition to the Vought A-7 Corsair II; and the A-4F came along after the designation change.XA-4AA-4AA-4BA4D-3A-4CA-4LA4D-4A-4EA4D-6A-4F