Unique, fixed Boeing customer codes have been used by Boeing Commercial Airplanes to identify the original customer for an aircraft since the advent of the Boeing 707.
An example would be a Boeing 747-400 ordered by British Airw
ays 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. Today, the permanent code denoting United Airlines is 22, not 34.
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: