D 53.5 mm.

This exceptional movement could be a special order for the King of Spain. This was probably the first time that a complete display of the equation of time was realized, using a separate dial with hour- and minute hands, on a portable watch. In order to achieve this, Thomas Mudge used both a worm gear and a differential gear.
Thomas Mudge was one of the most important watchmakers of his time. He is also considered the inventor of the lever escapement, from which almost all modern wristwatch escapements were developed.
Literature: "Watches" by Cecil Clutton and George Daniels, 1965, page 142.
Case: Bezel ring, signed: "Tho. Mudge London". Movement ring engraved: "Dedicado à D. Ramon Cabrera = Sus admiradores".
Dial: Front: Enamel dial with rotatable outer ring (annual calendar indicating the month, number of days, and signs of the zodiac); inner dial (hours in Roman numerals and minutes in Arabic numerals) with pink gold "sun hands". Back: Enamel dial with Roman numerals and minutes in Arabic numerals, "poker and beetle" hands, central second hand.
Movement: Highly complicated manual winding movement with cylinder escapement and central second. Differential gear for the equation of time, with worm gear, and repetition. Chain and fusee movement. Gilt full-plate movement, with early cylinder escapement, brass escapement wheel, and flat, polished steel balance. Signed: Tho. Mudge 422. Requires revision