Unique, fixed customer codes were used by Boeing Commercial Airplanes to denote the original customer for airframes produced as part of Boeing's 377 Stratocruiser and later 7x7 families of commercial aircraft until 2016.
[1] In 2016, Boeing announced that they would no longer apply customer codes to any aircraft produced after a certain point, which would lead to their designators being the "generic" type for the model.
Exceptions existed, such as in some cases if an airline cancelled or sold their order before Boeing had commenced building the airframe, the customer code would be changed to that of the new purchaser.
Also, 2 747-200Bs purchased by British Airways were sold while under construction, to Malaysian Airline System and remained 747-236Bs.
Another case is a situation where Boeing sells experimental aircraft to an airline, one example being the first two 777-300ERs built initially as experimental aircraft for Boeing which were subsequently sold to Japan Airlines had their codes changed from 777-300(ER) to 777-346(ER).