Rolex Sea-Dweller "Double Red"

The very first production Sea-Dweller was the Double Red Sea-Dweller (DRSD). The nickname refers to the two red lines on the dial, which set it apart from the handful of prototypes that feature just one line of red text. Those are, as you'd expect, called "Single Red" Sea-Dwellers. Interestingly, the helium escape valve is not a defining feature on all those prototypes, but it became the signature of the Sea-Dweller family as we know it today.

In comparison to the standard Submariner of the time, the DRSD and its 40mm steel case could be distinguished by the obvious valve on the left caseband. It also offered a date display at three o'clock, crucial information for divers measuring their lengthy stays under water :D

Interestingly, the plexi did not feature the usual cyclops over the date window, as it would not have sustained the underwater pressure (at the time the cyclops was simply glued onto the crystal). Instead there were thick domed plexiglass crystals that collectors today really like.