This particularly small size eight-day marine chronometer has an unusual format. The escape wheel, detent and balance wheel are mounted in a sub-frame assembly, the top-plate of which is secured to the top-plate of the movement. The spring detent is set in a keyhole recess and is positioned above the plane of the escape wheel teeth; it therefore has an inverted locking jewel and very much resembles the format of an Arnold escapement spring detent. The train carries an intermediate wheel between the fusee great wheel and the centre wheel which gives the eight-day duration. The wheel work and pinions are of very high quality finish with six-armed crossings, the escape wheel has four-armed crossings and is positioned almost within the thickness of the main top-plate.

A similar chronometer Barraud No. 2/670 but with a 24-hour dial is in the British Museum collection, Catalogue of Watches in the British Museum VI op. cit., illustrated in these plates is an identical movement to that of No. 999, the catalogue entry states that the style and finish of the platform and escapement suggests the work of Robert Pennington.