http://www.fordgarage.com/pages/ABenginenumbers.htm
"As shown above in the 1928 Model A Ford Instruction Book, Ford did in fact specify the engine number to be the 'serial number' of the vehicle, during the Model A (and B) era, and throughout the 1930's. The Specifications and License Data page above in the Instruction Book is quite clear and specific about this.
It is popularly but incorrectly claimed that the serial number of the vehicle was a frame number. Actually, not all Model A vehicles even had a frame number. A great many did not have a frame number stamped, and it varied depending on when and which of the more than two dozen assembly plants completed the vehicle final assembly. There is no Ford literature available indicating a frame number was ever intended for any primary identification of a vehicle by Ford."