Re:AAF Special projects Numbers (AAF Spec. Proj. No.) P-51D Mustang
While this is a bit after the fact, here's my 2 cents on Project Numbers:
1) Terminology - It is properly called a Project Number, not a Special Project Number. Special Project numbers were assigned to operational plans such as Project 9, which became Operation Thursday, the aerial invasion of Burma. A Project Number, as given in the data block stenciled on the nose of an aircraft, was a number assigned to a specific theater request for materials and equipment. Each request allocation was assigned a Project Number.
2) A Project Number was not specific to an individual aircraft, but it applied to a group of aircraft, as well as spare parts, personnel equipment, and so on. For example, all L-5 aircraft and equipment assigned to the First Air Commando Group Liaison Squadrons that were sent to India for the winter 1944 offensive in Burma bore Project 90689. All aircraft later sent to that theater as replacements for those units bore the code 90690R (r = replacement).
3) The Project Number was typically assigned long before the aircraft left the factory, and often before they were even built. You will find this number on the Individual Aircraft Record card. Besides appearing on the noses of the airplanes and on the IARC's, it was also stenciled on shipping crates, equipment bags, and so forth. The project numbers indicated the destination theater and receiving command, as well as the route by which the aircraft were delivered. In other words, it was a military shipment code that enabled a piece of equipment to get from point A (factory) to point B (final destination). Crews that flew the larger airplanes overseas were also assigned to the same project number.
4) Quite often, project numbers changed due to changing tactical and strategic plans, and high priority requests often caused equipment to be diverted. I have seen IARC's with 4 and 5 changes written on them. Sometimes this occurred after an aircraft left the factory, in which case the number stenciled on the nose didn't match the actual use of the aircraft. The projects that equipment was assigned to was determined by a central planning committee, the name of which escapes me at the moment. The War Production Board, or something of that nature.
5) No official lists of project codes seem to have survived. What is known about individual project numbers for aircraft has been compiled through patient research. I'm not a P-51 guy so cannot tell you much about their project codes. If you want to know about Stinson L-5's, I can tell you every project number they were assigned to.