The present watch is a transitional model between the GMT Master first series reference 6542 and the succeeding reference 1675 as it features still the early black lacquered dial with the gilt printing instead of the matte dial with silvered designations. It is furthermore fitted with the pointed crown guards whereas subsequently produced ref. 1675 models show the rounded shoulders. For this reason, the present model is also known as the "Cornino", meaning "small horn" GMT Master.
Due to the extraordinary success of the Rolex Sports models in the 1950s, it came to no surprise when the world’s largest airline Pan Am commissioned a specially designed watch allowing their pilots to keep track of time in two locations.
With the beginning of transatlantic flights with jet aircrafts in 1956, travellers appreciated the fact that the journey times were reduced from 13 hours to 7 but suffered from new phenomena, the jet lag. Worried about the possible effects on their pilots and, after researches, Pan Am management advised them to keep on "home" time while away from base. As at the same time they needed to know the local time and consequently, a joint task force from Pan Am and Rolex was assigned. The team devised a watch with an additional hour hand, which revolved once every 24 hours and a rotating bezel marked with those same 24 hours. The model was named "“G.M.T.-Master" after the world's standard time, the Greenwich Mean Time.