Relationship Between Block Numbers, Project Numbers, and Contract Numbers

[Old MacDonald]

After considerable research, including at NARA, I've not found anything concrete that addresses the relationship between USAAF aircraft type Block Numbers, Project Numbers, and Contract Numbers.

Has anybody seen such a technical analysis or description?

Conventional wisdom seems to have always been:

• Block Numbers (B-24D-45-CO, P-51C-5-NT, etc.) designated significant airframe, engine, or equipment changes from previous production,
  
• Project Numbers designated the combat group, numbered air force, or theater to which the aircraft was to be assigned, and
  
• Contract Number (actually the contract option or batch of the basic contract) was simply an accounting convenience.

Aside from the obvious fact that preplanned destinations and/or configurations did not always stand the test of "current needs of the AAF," there appears to be some evidence the conventional wisdom about what these three numbers meant may be incorrect or at least insufficient. While I've found no written evidence, analysis of thousands of photos shows there must have been some different system at work than what we've always assumed.

For the B-24, at least, there's some evidence the Block Number may have indicated a specific set of physical attributes, which may or may not have superseded previous configurations. In other words, it appears the Block Number did not designate sequential improvement of equipment fit, airframe configuration, or engine type, but rather a group of attributes, each of which may or may not have appeared in earlier Block Numbers.

Since Contract Numbers (designating a small-ish number of serials to be produced under that option or batch) constituted subsets of a Block Number (I've never found a contract option that spanned two or more Block Numbers), it's reasonable these contract options also designated something meaningful.

The contract options included varying numbers of serials, so it does not seem they simply designated "more" airplanes to be produced--it appears the airplanes in that option were to be built for some specified purpose beyond the simple need for "more" of that aircraft type.

My knowledge of this situation is more anecdotal than analytical, so I'll appreciate any confirmed info you guys can provide.