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 1530 perpetual movement.

Mechanism: Rolex originally launched the Calibre 1530 as a 17-jewel movement. It went through a number of significant upgrades itself during its successful run before retiring in 1965. They also released it in 25 and 26 jewel versions. Its initial butterfly rotor graduated to the familiar half-moon type. Also, they replaced the brass colored gears with the red, Teflon-coated variety we generally see today.

While extremely precise and reliable, Rolex considered the Caliber 1530 very much a workhorse caliber. Therefore very few were submitted for chronometer certification. Instead, Rolex fitted the movement into several of their non-chronometer models, such as the Air-King and the earliest versions of the Submariner, watches they initially deemed not to need mechanisms that had passed the rigorous COSC tests.

However, ask any watchmaker today what in their opinion is the best movement Rolex ever produced, and many will still say the Cal. 1530. Its lack of certification was down to its relative difficulty in regulating consistently rather than its overall accuracy. Before the innovation of Microstella screws were first introduced in 1959 on the Cal. 1565, the previous screw balance was far trickier to adjust, especially considering the volume of movements Rolex produce.

Features List: Self-winding movement, bi-directional winding; Breguet hairspring; Escapement: Stone lever; Yoke winding system; Screwed balance; Shock protection KIF Flector.

Family Notes: Rolex presented the first very Explorer in 1953 to commemorate Sir Edmund Hillary and Tenzing Norgay’s pioneering climb to the 8,848-meter summit of Mount Everest, the earth’s highest mountain. Over 60 years later, the Explorer still remains a very popular choice among Rolex’s collection of sports watches.

The Explorer was based on the already existing and sturdy Bubbleback series of watches, which has been modified by Rolex for its more audacious clients. In 1952, the references 6098 and 6150 (both using the A296 movement) were in fact the first prototype watches to be used by climbers; they were watches with white dials and leaf shaped hands, which didn’t bear the name Explorer yet. In 1953, these models became the references 6298 and 6350, when they were fitted with the typical Explorer-type 3-6-9 dial and later with the Mercedes hands. Before Hillary and Norgay climbed the Everest, Rolex didn’t name their watches Explorer. Although Rolex registered the Explorer trademark on January of 1953, the Explorer gained its name after Hillary and Norgay climbed up the Everest and “Explorer” began to be added on the Ref. 6350, at the end of 1953.