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This chronometer is most likely one of 5 that Hamilton produced in 1955 for the Seismic Laboratory of the California Institute of Technology*. The serial number of this example is 2E11659. It is based on the model 21 but has additional mechanisms for both minute and hour break circuits. The pieces were used to improve correlation between seismometers at distant locations and therefore improve the ability to localize the seismic event.

The circuits are implemented with modified spring detents made from the standard ones used for timekeeping. Each of these is jeweled, so the actual jewelling on the selective break circuit chronometers is 16 jewels. Hamilton produced chronometers with a single break circuit starting in 1945. In 1956 they had produced 201 single break and 5 selective break chronometers (more may have been produced later).

It is possible that the outer carrying box is a later addition, but the chronometer, tub and box all appear to be original with all serial numbers matching.

*Historical information on the chronometer is from Marvin Whitney's book Military Timepieces published by the AWI Press in 1992.