List of Boeing customer codes
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. == History == Boeing first used customer codes for the 377-10 Stratocruiser.
Source: Wikipedia — List of Boeing customer codes (CC BY-SA 4.0)