In 1954, Rolex launched the GMT-Master (model 6542), after a request from Pan American Airlines for a watch that “could simultaneously display the exact time in two different time zones.” The watch was an instant success, and was adopted by Pan AM, as well as numerous other airlines worldwide, as their official timepiece.
The earliest versions are easily identified because they are not marked ‘GMT-Master’ on the dial, but instead have the depth rating “50m = l65ft” printed in red above the 6. Early versions also featured an acrylic Bakelite bezel, which had a darker finish, so as not to be reflective into the pilot’s
eyes. This bezel was found to be inferior, because it had a tendency to fracture when in very warm environments. It was quickly replaced with a metal bezel, making these early
models quite desirable for collectors. Another rarity for the early GMT was the presence of a
white dial. Apparently, Pan AM ordered a small number (probably less than 200) of watches with this configuration, to differentiate between watches intended for air crew and for ground staff.”—thus, black dials were issued to air crew, with white dials issued to ground staff.4’
In the early I 960s, Rolex changed the color of lettering Early GMT (model 6542) with Bakelite printed on the dial from gold to white, and with it the date bezel. Note the tiny arrow on the tip of wheel numbers changed from ‘alternating’ redlblack to all black. By the mid 1 960s, the extra function 24-hour hand
switched from a small arrow on the tip, to a larger arrow. Around the same time Rolex changed the wording at the bottom of the dial from Swiss to Swiss T<25, signifying
the switch from Radium to Tritium as the luminous material used on the dial and hands.
Like the Submariner, the early GMT models did not have crown guards to protect the winding stem—this feature was added in the early 1960s (on model 1675), and a more rounded version was outfitted in the late I 960s. Also on the 1675, the wording “Official Certified Chronometer”
printed on the dial, was first changed to “Superlative Chronometer Officially Certified.”
Another feature missing from early models was the cyclops lens, positioned over the date aperture. Apparently, this was an option until the early 1960s, but very
few made it out of the factory without it, making this a bit of a rarity. GMT-Master ii (model 16710). Circa 2002 In the early 1970s, Rolex outfitted a special version of the 1675 with a Jubilee bracelet—making it the only sports model to be factory-fitted with this bracelet design. At this time, Rolex alsobegan offering the bezel with a black insert. Around 1976, the ‘hack’ feature was added, and in 1981 the 1675 was renumbered as the 16750 when it was fitted with the 3075 movement, and with it the ‘Quick Set’ feature and sapphire crystal upgrade. The 16750 was quickly replaced by the 16700 GMT-Master, and 16710 GMT-Master II, which utilizes the new 3085 movement.
The new GMT-Master II is now capable of one-hour jumps, forward or backwards (without affecting theminute or second hands), and allowing more flexibility in viewing any two time zones simultaneously.


Basic Operation of the 24-hour Hand & Bezel
(GMT-Master, GMT-Master II and Explorer II)

The Rolex GMT models utilize a special 24-hour hand and bezel, allowing the wearer to tell the time in two different time zones. The operation of this feature is as follows:
The 24-hour bezel is divided into two equal parts which are color coordinated (on some bezels). The time 06:00 to 18:00 (6AM to 6PM) is colored red to represent ‘day’. Whereas, 18:00 to 06:00 (6PM to 6AM) is colored blue (or black on some bezels) to represent ‘night’.
The 24-hour hand points to the appropriate 24-hour time represented on the bezel. For example, ten past ten, with the 24-hour hand pointing to the “10” on the red portion of the dial, tells us that it is 10:10 AM. Whereas, if the 24-hour hand was pointing to the “22” in the blue (or black) portion of the dial would indicate 10:10 PM. Thus, the bezel expresses the time in 24 hour “military time”.

GMT-Master II displaying the
time ‘ten past ten’ (AM)
Notice the position of the
24-hour hand.

(GMT-Master): With the bezel in the ‘zero’ or ‘neutral’ position, the triangle on the bezel i5 aligned with the triangle on the dial (i.e. 12 o’clock position). From here, the time reads ten past icr in the morning (as indicated by the 24-hour hand).
Lets say you are in New York (Eastern time zone), and you want to set the 24-hour hand to indicatc the time in California (Pacific time zone). Since Pacific time is 3-hours behind Eastern time, it would make it ten past seven in the morning in California. Simply turn the bezel until the number ‘7’ faces the 24-hour hand. Now the hour/minute hands indicate New York time, while the 24-hour hand indicates California time.

(GMT-Master 11): In addition to the function listed above, the GMT-Master II has an additional convenience feature. This model allows the regular hour hand to be set to a different time zone without adjusting the 24-hour hand and bezel, or affecting the watch’s accuracy. This is done by unscrewing the winding crown to the first notch (position 3). In this position, the watch operates normally and the time does not stop. Now, by slowly turning the crown clockwise or counterclockwise will ‘jump’ the hour hand one hour at a time without affecting the operation of the minute or second hand. Therefore, using the example above, the wearer could simply turn the hour hand back three hours (to ten past seven AM) and thus the watch would indicate the time in California (Eastern time).

Explorer II with 24-hour hand and bezel.

(Explorer II): While the Explorer II does not have the rotating
bezel like the GMT models, it does have the same 24-hour hand and
‘jump’ hour feature as the GMT-Master 11. Therefore, the operation
would be identical to that listed above.

Information found from “The Rolex Report”, revised and expanded 4th edition by John E. Brozek.