Sea-Dweller 16600 Comex Mark 1

The Sea-Dweller was developed in the late 1960s in partnership with COMEX, the French deep-sea diving company, for saturation divers who spent extended periods in hyperbaric chambers breathing helium-oxygen mixtures. The helium escape valve — a Rolex patent that allows helium molecules to exit the case during decompression without blowing off the crystal — was the Sea-Dweller's defining innovation and the feature that separates it from the Submariner. Rated to 1,220 metres (4,000 feet) in its current form, the Sea-Dweller has always been the Submariner's more capable, less glamorous sibling — a watch designed for professionals who needed more than 300 metres of water resistance and weren't concerned with whether it had a Cyclops lens.

Introduced in 1988, the Sea-Dweller 16600 is the heavy-duty brother of the Submariner 16610 of the same era. Compared to the Submariner, its case -now fitted with a helium escape valve- is slightly thicker. The glass does not feature the cyclops. The 16600 was replaced by the Deepsea 116660 in 2008. The pictured watch was made for the COMEX diving corp. There were two batches of 16600 delivered to Comex; one in 1992 with straight COMEX engraving on the caseback such as the present watch, and another batch with rounded engravings in 1997.