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Naval aircraft designations coded the Type AC,number of types accepted from that manufacturer, and the manufacturer of that plane.
So an SBD was Scout Bomber (first type)Douglas. A DC3 was coded R4D = Transport (4th) Douglas.TBF= Torpedo Bomber (1st)Grumman. A license built Avenger by GM was a TBM. GM- made Corsairs were FMs. If the Bureau of Aeronautics accepted Army type AC for Naval service, they got navy designators. B-25 was the PBJ (Patrol Bomber, North American), B-24 was the PB4Y -- the U.S. Navy used 2 types of PB4Ys -- the single tailed "Privateer" for long range patrol bombing and anti-submarine warfare, and the twin tailed "Liberator" type for photo recon.
The system remained in effect until about '61 when MacNamera reformed both the Navy and Air Force nomenclatures into a new single system. Thus the McDonnel "Phantom" F4M became the F-4.