Originally Posted by
Alan Baker
Boeing allocated customer numbers to airlines buying its aircraft which applied to the aircraft throughout its life and which also applied to any other Boeing types the airline bought. Thus a 707-436 was built for BOAC and 36 was added to any other Boeing type bought by BOAC or its successor British Airways, hence 747-136, 737-236, 757-236 etc. 99 was the customer number for Caledonian Airways for the two 707-320Cs that they bought, hence 707-399C.
A slight digression, but Boeing finally junked their customer number system three or four years ago. Having never applied it to the 787, and abandoned it on the 747 with the advent of the 747-8, the remaining types in production (737NG, 777, etc) also stopped using the system at a given point on each production line.
So, for example, Delta's N865DN (31976/6058) is a 737-932ER (32 being Delta's customer number), but N866DN (31977/6100) is certificated and registered as a 737-900ER, although some sources continue incorrectly to cite customer numbers for later NGs.