French 18K gold keyless dress watch with chronograph Circa 1915

Gold open faced lever pocket watch
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A very fine and rare French, 18K gold, keyless dress watch with chronograph, instantaneous 60-minute register and special balance, circa 1915. A polished case, the back engraved with a monogram. Gold engine-turned dial with dauphine Arabic numerals, outer minutes and fifths of a second track, subsidiary seconds at 6 and 60-minute register at 12. Blued steel Breguet hands. A gilt brass movement, 29 jewels, straight-line lever escapement, bimetallic compensation balance cut in the middle of the rim, blued steel Breguet balance spring, index regulator, chronograph with visible work, activated by a button in the winding-crown. Dial, case and movement signed.
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Diameter: 57 mm
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Signed: L. Leroy & Cie.  Paris
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The special balance is cut on each side at the mid-point. The reason for this unusual arrangement is that in this watch the balance spring must be almost self-compensating, one of the first using nickel and steel, therefore the bimetallic arms do not need such a large arc of expansion.