"The Hamilton 4992B was a Master Navigation Watch used by pilots for celestial navigation during WWII. These watches were used both my the Army Air Force and the Navy at that time. The watches were still used well after the war. Knowing the correct time is important in navigation because an error of only 4 seconds causes an error in position of up to 1 nautical mile. They had a 24 hour dial and were set to Greenwich Mean Time (or Greenwich Civil Time), because the navigational tables are based on GMT, also called Civil. The watches also had a hack feature, which stops the watch when the stem is pulled out. This allowed the pilot or navigator to set the watch to standard time before a mission." Don Dahlberg, 4/21/2011 Message Board thread.
For a brief discussion of the Hamilton Watch Co. grade No. 4992B (and information on more resources), refer to the Message Board thread entitled Hamilton Pocket Watch and to the Hamilton Military Watches Encyclopedia article.