This chronometer has its original box.
During the Battle of the Atlantic in WW2 many hundreds of Allied ships were being lost to German U-boat attacks. To begin with Allied shipping carried mostly British and Swiss-made Marine Chronometers for navigation, usually more than one per ship. By 1941 it was realised that the stock of these instruments was not sufficient to cover losses, so a solution was sought from US watch and clockmakers. The only design to meet the Navy's stringent tests was the Hamilton Model 21, made using the mass-production methods that had made America famous, such as the Model T Ford and the Colt 45, not to mention the extremely successful watchmaking industry.
Production of this chronometer continued until 1960, and it is now prized among collectors for its accuracy.
It has one unique feature - the locking device for the balance wheel which saves the need for careful wedging of every other make for travel beyond its use in gimbals at sea.