The GMT-Master was created by Rolex in collaboration with the Pan-Am Airline company who, in the 1950s, flew non-stop across the Atlantic. To prevent their pilots and crew becoming jetlagged, Pan-Am convinced the Swiss company of the necessity of knowing the local time at departure and arrival locations simultaneously. Thus in 1954 the GMT Master reference 6542 appeared on the market; the watch allows a second time zone to be displayed thanks to an additional hand. This additional hand makes a complete circle in 24 hours and indicates the exact hour on the bezel graded from 0 to 24. Reference 6542 has a three piece Oyster case guaranteed waterproof to 50m, black dial, luminous tritium indices, bordered chapter ring, Mercedes hands with the additional hand carrying a small luminous triangle at the end. The graduated rotatable bezel in Bakelite is engraved with 24 hour markers on the lower part, filled with luminous material. The bezel has two colours: red for daytime hours and blue for night-time hours. The Bakelite bezel, although aesthetically very pleasing, was eventually replaced by a more resistant aluminium insert. All GMT-Master movements, from 1036 to 1065 have been certified chronometer, demonstrated by the words “Officially Certified Chronometer” on the dial. Generally the reference 6542 is available in steel and, to a limited extent, in yellow gold, with an amber dial, appliqué indices, metal leaf hands and brown Bakelite bezel. In 1959 reference 6542 was replaced, with some changes, by the reference 1675. The GMT-Master 1675 is the only Professional model produced from the start with a Jubilée bracelet, instead of the usual Oyster bracelet. Amongst the changes are protective shoulders for the crown, new 1565 GMT movement, more obvious fluting of the bezel and the lugs have a more marked shape. After 1960 the new models underwent certain other modifications such as a change in colour of the date and the inscription on the dial, which became Superlative Chronometer Officially Certified. The chapter ring is still bordered, apart from some 1964 models with a rare underline dial. Underneath the 6 o’ clock hour marker SWISS became SWISS T < 25. Other modifications were made in 1967 when the inscriptions on the dial were changed to white on a matt black background and new models had a 24 hours hand with a larger and more legible triangle at the end. Starting from the early 1970s the option of a black bezel became available. Even the yellow gold versions underwent some changes, such as the addition of protective shoulders for the crown and the introduction of a black dial. The GMT-Master reference 1675 in steel and gold, like the gold versions, gained a gold/brown insert specifically for the “tiger’s eye” dial. In 1981 the last important change was made, the adoption of the 3075 calibre with an instantaneous date jump mechanism. The new collection introduced five-number references: 16750 (steel), 16753 (steel/gold), 16758 (gold) and 16760 (steel). The Rolex GMT Master model 16750 acquired a new glossy dial and luminous indicators but kept its acrylic crystal. Some recent GMT-Master references: 116710 LN steel, 116713 LN steel and yellow gold, 116718 LN 18kt yellow gold, 116759 SARU 18kt white gold, 116759 SA 18kt white gold, 116759 SANR 18kt white gold, 116758 SARU 18kt yellow gold, 116758 SA 18kt yellow gold, 116758 SANR 18kt yellow gold. Latest versions have a ceramic bezel with the model name and the 24 hours hand in green.