Sea-Dweller 16660 Matte Mark 2

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.

The Sea-Dweller 16660 'Triple Six' was the first Sea-Dweller to be fitted with a sapphire glass. The first series used matte dials (divided in Mark 1 and Mark 2 variants) while the second series used the newer gloss dials. It was replaced by the 16600 in 1988. The present watch has a Mark 2 matte dial, most easily identified by the base of the 'F' in 'Feet' reaching to the 'V' of 'Superlative'. On the Mark 1, the base reaches to the 'E'.