Unique, fixed Boeing customer codes have been used by Boeing Commercial Airplanes to identify the original customer for an aircraft for all Boeing 7x7 aircraft, up to and including the Boeing 777, as noted below. The codes were announced at an ICAO congress in 1956 and were used by Boeing until the summer of 2016.
An example would be a Boeing 747-400 ordered by British Airways would be a Boeing 747-436 (36 being the customer code). The codes do not change if the aircraft is subsequently sold as they reflect the original configuration of the aircraft.
Before the 707, Boeing used a generally similar system to identify the presence of detailed variations or options requested by particular customers, but the codes were not customer-specific. A Boeing 377 Stratocruiser built to the requirements of United Air Lines, for instance, was designated a Model 377-10-34. In the 7x7 series, the code denoting United Airlines is 22, so a 747-400 built for United Airlines is a 747-422.
In 2016, Boeing announced the discontinuation of customer codes for 7x7 models still in production (up to and including the 777), with the effective line number indicated beyond which (inclusive) a customer code will not be given.
Additionally, customer codes were never used for the Boeing 787 Dreamliner and Boeing 737 MAX, and were not planned to be used on the Boeing 777X either.
List
The order of codes has not been sequential, as the first 707 was designated the 707-120 by Boeing, so the customer codes started at 21:
Airlines in italics are defunct.