in 1967, Rolex launched the Oyster Perpetual Sea-Dweller, which was water-resistant to a depth of 2,000 feet. It was actually requested by the Comes S.A. industrial deep-sea diving company so the watches are tested to withstand the pressure that the divers do. The reason why it is called a “rail” dial is not clear, but it is used to describe Rolex dials on which the letter C’s from the words “chronometer” and “certified” are lined up in a straight line vertically. Rail dials were only made for two years between 1977 and 1979.
Read the full story here :: Rolex Sea-Dweller Reference 1665 With ‘Rare’ Rail Dial