Contrary to the artist's inscription stating that the enamel is 'After Miller', the head is close to the 1690 full-length State Portrait by Sir Godfrey Kneller at Windsor Castle showing William III in State Robes (405675). Half-length variants of Kneller's portrait showing the King in armour are known, but they invariably show him wearing a lace jabot, and the breastplate differs in detail. The closest comparison is to a very large rectangular enamel, measuring 130 × 103 mm (private collection), which has been confirmed as dating from the early eighteenth century. It was formerly in the Royal Collection (Enamels 1877, I, p. 309) and is the most likely source for the present miniature. That enamel can be attributed to Charles Boit, enameller to William III, and it is not clear why Simpson should have inscribed his copy as 'After Miller' unless he also made use in the production of his enamel of a mezzotint by Andrew Miller (d. 1763) of William III after Kneller.

John Simpson (1811-after 1871) was trained as a porcelain painter at the Derby porcelain factory before moving to London in 1845. He soon established a practice as a miniature painter specialising in works on enamel, and fulfilled the first of many commissions for Queen Victoria in that year. He is last recorded describing himself at the end of his career as a 'photographic artist' (1871 census return); the date of his death is not known.

Signed, dated and inscribed on the counter-enamel in red paint: King William III. / Simpson 1846 / After Miller