At the outbreak of W.W.II the United States Navy required a vast quantity high quality chronometers. Hamilton Watch Company was the only firm able to meet the requirements of the War Office, and produced enough units to meet US wartime demand within the 18 months requirement.

 

The model 21 was technically one of the great achievements in horology. The balance and hairspring assembly were radical from the traditional chronometer design and resulted in superior time keeping rates.