Hamilton 1941-42 WWII Military 24 Hour Pilots Pocket Watch.
The AN5740-1 was commissioned by the United States Government at the beginning of World War II and built for the Army Air Corps. There were strict requirements for this timepiece because the brave soldiers in the skies needed to know the accurate time.
Three of the Great American Watch Companies were up to the task. Elgin from Illinois, Waltham from Massachusetts, and Hamilton from Pennsylvania all built mechanisms that met these specifications. To make the mark, all three companies decided to retrofit their most valuable and accurate Railroad Grade movements.
The dials were all standardized to be black with simple, white 24-hour markings. Each had a set of white hands, and a long sweeping second hand with a hacking feature that allowed the timepieces to be used not just as pocket watches but as stopwatches.
Called the Master Navigational Watch, these pocket watches were used by the Navigator on Bomber aircraft like the B-17, B-24, and B-29. Each dial was marked with "GCT," known at the time as Greenwich Civil Time because that was the standard European timezone. Now called UTC or Coordinated Universal Time, it relates to the time at 0 degrees Longitude which is the location of the Royal Observatory in Greenwich, England.
When determining your location by using the stars, if your watch is off by about four seconds, you could be as much as one nautical mile off course. For that reason, these pocket watches needed to be extremely accurate and were even mounted inside gimbal systems to protect them from the aircraft vibrations.