Rolex launched the Oyster Perpetual Sea-Dweller in 1967, a watch that was water-resistant to a depth of 610 meters (2,000 feet). The model was requested by the Comex S.A. industrial deep-sea diving company and was therefore fitted with a helium escape valve.

Professional divers can spend many hours and even days in high-pressure decompression chambers

Helium is the lightest element with the smallest atoms. Professional divers spend long periods surrounded by helium-rich gas as they breath a mixture of helium and oxygen rather than normal air, which is nitrogen and oxygen as the nitrogen can lead to nitrogen saturation of the blood (known as “the bends”).

The small helium atoms under high pressure can work their way into watch cases. As the diver ascends or decompresses, the helium expands. Unless released from the case, the expanding gas can pop the crystal off.

Rolex Sea-Dweller Reference 1665 with “rail” dial

The collector community has christened vintage Rolex models with a great many nicknames. These include monikers that, for example, allude to superstar salad dressing manufacturers (Paul Newman dial), fear-inspiring eight-legged arachnids (spider or spider web dial), and even a U.S. state known for its “governator” (California dial).

I believe that many of these nicknames for rare Rolex dials were coined from 1990s through the early 2000s by influential collectors and vintage watch dealers.

Rail dial with the Cs of “chronometer” and “certified” lined up vertically; post-1975 (or thereabouts), the Rolex Sea-Dweller Reference 1665 had white text only on the dial

While the exact origins of the word “rail” are not clear, this name is used for Rolex dials on which the letter C within the two lines stating “superlative chronometer” and “officially certified” line up as straight as train tracks.

Since the rail dials were only made for a short period of two years between 1977 and 1979, they are considered rare. However, it is important to remember that using the word “rare” is really is an oxymoron when talking about Rolex because there are just so many. Rolex probably produced thousands of watches with “rail” dials.

Rolex Sea-Dweller Reference 1665. Note that the two Cs do not line up vertically, this is not a “rail” dial (photo courtesy Bert Buijsrogge)

Rail dials can also be found on Rolex GMT Reference 1675 and the Explorer II Reference 1655 in addition to the Sea-Dweller Reference 1665 shown here. As they were only in production for a relatively short period of time, they are in high demand among collectors. This, of course, means that they command a higher price than their “regular” counterparts.

Rolex Sea-Dweller Reference 1665 with “rail” dial on the wrist

The watch is really comfortable to wear despite its hefty size and, in particular, its height of 17.7 mm. It is actually surprisingly light, weighing less than 129.58 grams.

Non-“rail” dial, note that the Cs of “chronometer” and “certified” do not line up vertically; pre-1975 (or thereabouts) versions of the Rolex Sea-Dweller Reference 1665 had two lines of red text, which earned it the “Double Red” moniker

The Oyster Perpetual Sea-Dweller Submariner 2000 was introduced in 1967 with two lines of red writing on the dial, which led to being unofficially referred to as the “Double Red.” Around 1975, the red words on the dial disappeared, leaving all the verbiage white, and the word Sea-Dweller was added, naturally also in white.

Considering how Rolex watches are perceived nowadays, it is quite interesting to note that in the 1970s a Rolex Sea-Dweller was a tool watch, and not a luxury timepiece. Although today they are still tool watches, they are something else as well.

To my knowledge, the Rolex Sea-Dweller was the first diving watch outfitted with a helium escape valve in 1967, however Doxa claims to have offered the first to the public with its Conquistador dive watch in 1969 (the Sea-Dweller was initially only offered to commercial divers). And it was Rolex that patented the function.

For more information, please visit www.rolex.com/watches/sea-dweller-4000/m116600-0003/magazine.html.

Quick Facts Rolex Sea-Dweller Reference 1665
Case: 40 x 17.7 mm, stainless steel, with superdomed Plexiglas T39 and helium escape valve
Reference number: 1665
Movement: automatic Rolex Caliber 1570 (technically Caliber 1575 according to Rolex spare parts catalog R6), 26 jewels
Approx. value: today €10,000; the original price was just a few hundred dollars