The present watch is a nice example of the first GMT-Master model reference 6542. It features the sought after bakelite bezel and black lacquer dial.
With the beginning of transatlantic jet flights in 1956, travellers appreciated the fact journey times were reduced from 13 hours to 7, however they suffered from new phenomena, jet lag. Early airlines like Pan American worried about the possible effects of jet lag on their pilots and after research, the management advised them to keep on "home" time while overseas. Because crews needed to know the local and their "home"time, Pan Am and Rolex worked together to create a timepiece with an additional hour hand. The model was named GMT-Master after the world's standard time, Greenwich Mean Time.
The model is described and illustrated in 100 Superlative Rolex Watches by John Goldberger, pp. 228 & 229.