Where the Submariner is a diver's watch, the Sea-Dweller was built for saturation diving — living for days in a pressurised helium atmosphere. That produces a problem no ordinary case has: helium works its way in, and on ascent expands faster than it can escape. Rolex's answer was the helium escape valve, which is why this reference exists as something other than a larger Submariner. Water resistant to 1,220m, with the date and Cyclops. What matters on examination: that the escape valve is intact and has been serviced by someone who understands it; original bezel and insert; case sharpness, which the thicker Sea-Dweller lugs make it easier to hide a polish on; and bracelet condition including the extension link. Confirmed through the desk. Maker: Rolex. Reference: 126603-0001. Available through ELITA's private desk — verification review, discreet handling and storage. Price on request.