According to John Simonton, early PAiA products were numbered so that the first number was the month the item was released and the following two numbers would be the year (See Vail, p. 220, first edition). For example, the early PAiA programmable drum machine was PAiA 3750, therefore it was released March, 1975. This system was stopped in 1976 or 1977.
Vail, Mark. 1993. Vintage Synthesizers: Pioneering Designers, Groundbreaking Instruments, Collecting Tips, Mutants of Technology. Miller Freeman Books.