Marvin E. Whitney, Military Timepieces, AWI Press, 1992, pp.171-172.
Hamilton produced only 27 4-Orbit chronometers, between May 1945 and December 1956. Known as Model 221, their serial numbers were prefixed '4E'. The Navy contracted for ten of the chronometers but World War II ended shortly after the first was produced and the contract was cancelled. Most of the chronometers were subsequently sold to Northwest Instrument Co. of Seattle.
The 4-Orbit or Individual Orbit chronometer gives separate orbits for hours, minutes, seconds, day of week and up/down indication. It was designed specifically for use in the Pacific, where crossing the International Date Line made it essential for a navigator to know the day of the week at Greenwich (the Greenwich Date), enabling him to enter the correct date in the Nautical Almanac and thereby eliminate possible errors arising from local calendar changes at the International Date Line. The day of week hand is therefore set to the Greenwich Day.
The prototype model and several of the early-numbered chronometers had '00' at the top of the dial, as on the present example.