Reference Number: 1665 Year: 1981-1982 (Approx.) Case Size: 40mm Materials: Stainless Steel Functions: Time w/ Running Seconds, Date Display, Helium Escape Valve Dial: Matte Black w/ Luminous Hour Markers Luminous Material: Tritium Bezel: Bidirectional, Aluminum Insert w/ 60-Minute Scale Crystal: Acrylic (Domed) Movement: Rolex Caliber 1575 Water Resistance: 610 Meters / 2,000 Feet Strap/Bracelet: Oyster Bracelet (Stainless Steel) the Sea-Dweller features increased water resistance and a helium escape valve, which enables it to let trapped helium molecules safely exit the case without damaging the watch. This fascinating piece of technology was developed by Rolex during its early partnerships with the U.S. Navy and the French deep-sea diving company COMEX, and it has been the defining feature of the Rolex Sea-Dweller collection ever since. Not only is the reference 1665 the very first Sea-Dweller that Rolex ever produced, but it also represents the first time that a helium gas escape valve ever appeared on a Rolex watch. In addition to being the very first Rolex Sea-Dweller, the ref. 1665 is also the only version of the Sea-Dweller to ever feature an acrylic crystal. All subsequent models feature flat crystals made from scratch-resistant synthetic sapphire, but the Sea-Dweller 1665 is fitted with a domed crystal made from thick plexiglass, which offers an unmistakably vintage overall look and feel. Beyond that, the reference 1665 is the only version of the Sea-Dweller to ever feature a 610 meter depth rating, and it also happens to be the only Sea-Dweller model that has a movement without a quickset date. After 1977, Rolex switched to a dial with all white text for the Sea-Dweller ref. 1665, and the “SUBMARINER 2000” line was eliminated entirely. Nicknamed the “Great White,” this style of Sea-Dweller dial remained in production until the reference 1665 was finally discontinued in 1983. With that in mind, not all of these Sea-Dweller 1665 dials are the same, and there exist numerous different variations, which have been categorized as Mark I through Mark 4.