Up to then, the production of marine chronometer timepieces used by mariners for determining longitude and fix their position by celestial navigation at sea, was a highly skilled mainly craft-based and hence expensive horology niche and marine chronometers used by United States ships were imported or used foreign key components. The U. S. Naval Observatory had asked American watch manufacturers in 1939 to participate in domestically mass-producing chronometers. Hamilton was provided with two Swiss Ulysse Nardin marine chronometers to examine. Hamilton successfully perfected the mass production process and parts interchangeability maintenance method for these specialized timekeepers. The Navy accepted its product in 1942. The industrial production methods enabled the company to produce marine chronometers and deck watches in large numbers to fill the navigational needs of the United States Navy, Merchant Marine, and other Allied navies as well. The Model 21 Hamilton Marine Chronometer for large vessels was built first and had a chain drive fusee, the second hand advanced in ½ second increments over a 60-second-marked sub-dial, and was kept in a traditional gimbaled double box for the express purpose of keeping the clock in a "dial up" position to minimize ship movements induced timing errors. In most respects, the Model 21 marine chronometer technically resembled the Ulysse Nardin design, except for a new balance design and the use of a pre-formed Elinvar alloy balance spring, to obviate the cumbersome spring adjustments previously necessary. Hamilton produced 8,900 Model 21 marine chronometers for the Navy, 1,500 for merchant shipping, and 500 for the Army during the war. The Model 22 Chronometer Watch followed it for smaller vessels. The Model 22 looked like a large pocket watch and had a traditional mainspring, available in a traditional gimbaled double box and also in a deck watch for larger ships for transferring time from the box marine chronometer(s) for position fixes. The Models 21 and 22 had a two-day power reserve and the movements of both were marked U. S. Navy Bureau of Ships. The Model 22 was also used by the U. S. Army and on the back of some, it is marked U. S. Army, but all the model 22 movements are marked U. S. Navy Bureau of Ships. The Model 23 was a 16 size chronograph pocket watch. The Model 4992b was in a 16 size case with a black dial. It was used as the pocket watch for the U. S. military, featuring a less accurate 21 jewel railroad grade movement.