Rolex, "GMT-Master". The first version of the GMT-Master was, like many of Rolex's other great "Professional" watches, the result of a joint development team. On this occasion it was the efforts of Rolex's research and development and Pan American Airlines pilots which resulted in the launch of the Ref. 6542 in 1954. These first watches were fitted with a resin bezel to reduce glare from the surface of the watch. This however was susceptible to changes in temperature resulting in the separation of the inset and the alloy coated revolving resin bezel, paint chipping due to moisture entering after slight separation, and the bezel and inset often flexed and cracked. Due to these faults there are very few remaining Ref. 6542's with resin bezels. From the launch of the first GMT-Master up to the launch of the GMT-Master II Ref. 16760 in 1985 there are three references, Ref. 6542, 1675 and 16750. Within these three references there were no less than 15 variations, some of these were no more than iconografic while others were to do with the case, addition of crown-guards, and the movement, calibres 1066, 1565 and 3075. Since the GMT-Masters launch it has been the watch of choice for commercial and private aviators the world over, as well as with astronauts, there are several watches at Rolex's headquarters in Geneva which were used as "Private Equipment" by astronauts on both the Mercury and Apollo programs. There were also a small number of watches made with fixed bar lugs, for the British Royal Air Force in the 1950's and 60's. Examples of GMT-Master watches are published in "Rolex, Collecting Wristwatches", by Osvaldo Patrizzi, Guido Mondani Editore, Genova, 2001.