Extremely rare Swiss 18K gold Chronomètre Royal watch Circa 1910

Gold ankerchronometer by Vacheron & Constantin
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A very fine, keyless-winding Swiss 18K gold pocket chronometer, Chronomètre Royal, with open face, circa 1910. Extremely rare in this size.
Round shape 18K gold case with on the back a bright décor, guilloché with a sunray motif, case number 215521, and completely signed and numbered. Signed enamel dial with Arabic numerals, outer minutes track, subsidiary dial for the seconds, and gold spade hands. Very fine anchor chronometer, inscribed Chronometre Royal, oversized chronometer balance, fine adjustment, work number 350660, completely signed and numbered.
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Diameter: 47 mm
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Weight: 77 g
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Signed: Vacheron & Constantin Genève Chronomètre Royal
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Chronometer
Etymologically, any instrument for measuring time. In actual usage, however, the word has been given another sense, a precision instrument. A very good watch is sometimes referred to as a regular chronometer.
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The Swiss Federation of Watch-Manufacturers’ Associations (F. H.) defines the chronometer as follows: a watch that has obtained an official rating certificate. In Switzerland, official rating certificates are issued only by the observatories at Geneva and Neuchâtel and by the SWISS Institutes for Official Watch Time-keeping Tests.