From Christies-
The Field Testing
Rolex began testing its watches in the 1920s amid all manner of extreme pursuits: In 1927, Mercedes Gleitze was supplied a newly-introduced Oyster for her attempt to repeat her swim across the English Channel (she quit after more than 10 hours in cold water; her Rolex didn’t); from 1927-1936, Sir Malcolm Campbell set nine land speed records wearing an Oyster; Rolexes were also worn by pilots setting long distance flying records in the late 1930s, and in establishing the 1955 record for “a double crossing of the Atlantic by a small boat under power.” The publicity was priceless but the feedback from these wearers was paramount.
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The Conquest
Rolex was heavily involved in polar and alpine conquests, from which the watchmaker learned a good deal that ultimately informed its Explorer line. For example, the Oyster case was strengthened to endure extreme pressures. Rolex supplied (or touted the use of privately purchased) Oyster watches that accompanied no fewer than 15 Himalayan and Indian expeditions between 1933 and 1953. Most notable was the summit of Mount Everest on May 29 1953 by a British-led expedition. At least one Oyster Perpetual chronometer (likely a 6084 or 6098) accompanied Sir Edmund Hillary and Sherpa Tenzing Norgay to the world’s highest point.
The Promotion
Three days after conquering Everest, the British crowned a new queen. Several months later, an ad in the November 1953 issue of Horological Journal announced: “The New Elizabethan Age Heralds a New Rolex Triumph. A watch to measure the adventurous hours of the new reign. The watch has been called the Rolex ‘Explorer,’ and it has been designed to withstand every conceivable hazard.” Touting Arctic oil lubricant (guaranteed fluidity to -40 F), antimagnetic qualities, and the legibility of its 3-6-9 dial, the ad boasted that the watch would "function perfectly under water to depths of at least 300 ft, and in the air to the fantastic height of 12 miles.”
The Name
The Horological Journal ad went on to note that the Explorer, specifically, had been “tested on several recent Arctic Expeditions.” Rolex had registered the name “Explorer” on January 26, 1953—four months before the successful Everest summit. But none of those expedition-supplied Rolexes (“Bubbleback” references 6084 and 6098) bore the Explorer name. It's unknown exactly when the word “Explorer” first appeared on dials, but Rolex definitely sought to connect the watch with its Everest participation. The multitude of Explorer references created following Hillary and Tenzing’s success have been linked to that singular event ever since.
The Lineage
This 1016 became the unmistakable Explorer standard but it took some time. Despite Rolex’s popularity in 1953 and the magnitude of the Everest achievement, the company wasn’t sure how to position its new Explorer. So it experimented. Examples include: dials with 3-6-9 configuration but without the “Explorer” signature (others had the reverse); chronometer and non-chronometer dials; time-only and date versions; Air King Explorers; white, champagne, and black honeycomb dials; and even 3-6-9 dials signed “Everest”. Very rare hybrids place the 3-6-9 Explorer-style dial (minus the word “Explorer”) on a 5512 or 5513 Submariner.