Rare French gold open faced verge watch and key Circa 1800

Gold verge and key with calendar, moon phases and lunar calendar 
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A very fine and rare French 18-carat gold open faced verge pocket watch and 18-carat gold watch key with day and date calendar, moon phases and lunar calendar, circa 1800. Watch made by Etienne Tavenier, key made by Louis Tavernier.
Watch: Plain case by Amy Gros, master mark AG, bezel release catch in the crown. Hinged and sprung gilt metal cuvette. White enamel dial with Roman numerals, outer minute track, winding arbor between 1 and 2 o’clock and original blued steel Breguet hands. Frosted gilt full plate movement, 40 mm, waisted cylindrical pillars, fusee and chain, verge escapement, three armed brass balance, flat balance spring, balance cock pierced and engraved with foliate initials ET, silver regulation dial. Cuvette and movement signed, case with Amy Gros master mark and French Hallmarks.
Watch key: Polished bezels, glazed on both sides, flat band, swivel loop. White enamel dial on the front with day (in black) and date (in red) indications, blued steel hands. The back with polished gold plate with moon phases aperture, enamelled moon disc, and age of the moon engraved on the border. All the functions advance by depressing a small lever protruding from the band each time the key is inserted into the winding square. Dial signed.
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Diameter: 51 mm
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Signed: Etienne Tavernier à Paris
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Etienne Tavernier (1756-1839) invented this type of watch key while working in the Rue de Bussy in Paris (1772-1804). Tavernier is also well known for making watch cases and keys for Breguet. The present watch key is signed L. Tavernier for Louis Tavernier, Etienne’s son.
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The Tavernier’s calendar key. This type of key is instantly recognisable to horologists as a ‘Tavernier’ key. It is a beautiful object and a clever invention that keeps the owner supplied with the correct date. Every day one’s watch requires winding, and whilst performing this task, the calendar in the key is automatically advanced by one day. When the key is placed on the watch’s winding arbor, the calendar is advanced by one day, and a steel rod is pushed out of the side of the key. The moon phase is advanced when the rod is pushed back into the case.