The earliest version (Model 6542) featured an acrylic Bakelite bezel with a dark finish. This finish was to ensure that the bezel did not reflect into the pilot’s eyes while he was flying the airplane. However, today it is almost impossible to find such a model with this original bezel as it was replaced with a metal bezel soon after the initial launching. This stylistic change was made because the original bezel could not withstand the warm climatic conditions. Another feature of the earliest version was that it sported a depth rating of 165 feet, emblazoned in red color, on its dial and not the name “GMT-Master.” In addition, none of the early versions used the Cyclops lens that was usually placed over the date aperture.
The earliest versions of the GMT-Master also were made available in two dial colors: white and black. It is intriguing to note that this dual color option actually was requested by the Pan American Airlines because they wanted black dial GMT-Master wristwatches for the air crews and the white dial for the ground staff.
The special 24-hour bezel is divided into two equal parts: the time 6:00 to 18:00 in red representing day while the time 18:00 to 6:00 in blue or black representing night. In some models, this division is represented by two different colors.
Article source: https://www.thesequitur.com/features-of-the-rolex-gmt-master-earliest-version-698178/
