Reference 5520 the ?Air-King GoldShell? appeared in the Rolex catalogue from 1974 until 1986. The present lot bears two reference numbers 5520 on the body of the watch which refers to the specific characteristics of this watch and Ref 1024 in the case back which refers to the series of references to which it belongs. The case bodies and backs were interchangeable at the factory. This watch is illustrated in ?Rolex Collecting Wristwatches? by Osvaldo Patrizzi 2001 Edition p 363.