Notes: Rolex associates the ref. 5501 with both the Air-King and Explorer models. Both variations of the reference number are indistinguishable save for the name inscribed on each dial. They also both feature a 34mm Oyster case topped with a time-only dial and equipped with a calibre 1520 or 1530 perpetual movement.

Mechanism: In 1963, in a curious and extremely un-Rolex-like move, the Cal. 1530 started its own process of being phased out. Rolex replaced it by the less advanced Cal. 1520. As the numbers suggest, it represented something of a backwards step for the usually progressive thinking company.

It was centered on the same architecture as the Cal. 1530, and shared an identical base plate. However, it had several key differences, many of which were designed to keep its manufacturing costs to a minimum.

Rolex only produced it with a stick regulator, rather than with the Microstella system that had found its way onto the departing 1530 by the end of its run. The Breguet overcoil of the majority of Rolex’s output was substituted for a traditional flat hairspring, and it was originally released as a 17-jewel movement, although 25 and 26-jewel versions joined the range later in the production cycle.

Family Notes: The Rolex Air-King history dates back to 1945. During this time, the Rolex Oyster Perpetual was becoming increasingly popular among combat pilots for its durable case and easy-to-read dial. Hans Wilsdorf appreciated the fact that many brave pilots opted for a Rolex timepiece and chose to honor their loyalty with the addition of the Rolex "Air Series", which also included the iconic Air-King