Thomas Mercer (1822-1900) was an English chronometer maker who devised a robust balance and helped restore and repair Harrison's chronometers. This example was probably made by his son, Thomas Mercer, Jr. (1876-1935). The U.S. Navy Bureau of Ships transferred it to the Smithsonian in 1953. It is a 56 hour instrument with a later pattern Earnshaw spring detent escapement, and indications for hours, minutes, seconds, and up and down.
Tony Mercer, Chronometer Makers of the World (Colchester, 1991), pp. 203-204.