English round brass case so called box chronometer Circa 1824

Marine chronometer with chronometer escapement
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An important marine numbered chronometer, the so called box chronometer, made by John Roger Arnold in London, England, circa 1824. The watch has a round open faced case crafted from brass with bayonet locking and a winding whole in the bottom. The original numbered dial has Roman hour numerals, a subsidiary seconds dial at six o’clock and blued steel hands. The extremely rare movement has an Arnold detent chronometer escapement with gold hairspring and experimental balance according to a patent 4531 from 1921 (only eight pieces are known) and a  diamond end stone.
Dial and movement have matching numbers and are signed.
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Diameter: 85 mm
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Signed: Jn. R. Arnold London Inv^tit et Fecit
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According to Hans Staeger in “100 Jahre Präzisionsuhren von John Arnold bis Arnold & Frodsham”, only eight examples of this extremely rare balance are preserved, illustrated and described on pages 387 and 388. A matching box chronometer No. 491 is in the “National Museum of American History”.