Interesting Swiss silver keyless lever chronograph Circa 1918

Silver 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, circa 1918. Frosted gilt movement, 20 jewels, straight-line lever escapement, cut bi metallic compensation balance, blued steel Breguet balance spring, swan-neck micrometre regulator, visible chronograph work, co axial chronograph button on the winding crown. Hinged silver cuvette. White enamel dial with luminous bold Arabic numerals, outer minute track and fifths of a second chronograph divisions, subsidiary seconds dial and original luminous blued steel skeleton hands. Solid, polished case, the back with Corps of Engineers inscription. Dial, case, cuvette and movement signed, case and movement numbered.
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Diameter: 52 mm
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Signed: Corps of Engineers U.S.A. Vacheron & Constantin Genève