Interesting Swiss silver chronograph lever watch Circa 1918

Chronograph made for the U.S. Army Corps of Engineers 
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A fine and interesting Swiss silver open faced chronograph keyless lever pocket watch made for the U.S. Army Corps of Engineers in 1918. Frosted gilt-brass movement, 20 jewels, straight-line lever escapement, cut bimetallic compensation balance, blued steel Breguet balance spring, swan-neck micrometre regulator, chronograph with visible work, the button on the winding crown. Hinged silver cuvette. White enamel dial with bold upright luminous Arabic numerals, outer Arabic minute and chronograph ring and subsidiary seconds, original blued steel luminous skeleton hands. Polished case. Dial, case and movement signed, case numbered.
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Diameter: 52 mm
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Signed: Corps of Engineers U.S.A. Vacheron & Constantin Genève