In 1773 Adam produced designs for an organ case to house an instrument built by John Snetzler. The earliest-known drawing for the organ is within the plan and elevations for the music room (SM Adam volume 40/71). The organ, for which Snetzler was paid £250, was installed in the room by 5 February 1775 when Benjamin Cooke, organist and master chorister of Westminster Abbey, gave a performance.

Adam’s fine case was executed by Robert Ansell in painted wood, gesso and ormolu and included life-size figures of the muses Terpsichore and Euterpe, which flank the pipes. The figures were modelled in plaster and fitted by Richard Collins in 1777 and gilt bronze knobs, miniature versions of Adam’s door furniture, were supplied by Edward Gascoigne.

As one of only two surviving organ cases designed by Adam the piece is of extraordinary significance. The surviving Kedleston organ, which predates Sir Watkin’s, has undergone significant alteration and the piece designed for the Earl of Bute was destroyed in a fire. The fate of Adam’s remaining organ cases, if ever executed, is not known.

The Williams-Wynn organ was removed from 20 St. James’s Square in 1864 by the 6th Baronet of Wynnstay, who spent little time at his London residence. At Wynnstay it was altered by Gray and Davison who inserted a ‘hydraulic blowing machine’, before the instrument was installed in the gallery of Wynnstay’s great hall. In 1946 Wynnstay, along with its fixtures and fittings, were sold to Lindisfarne College. Following a liquidation sale the organ was purchased in April 1995 for the National Museum of Wales.

Significantly, the organ case retains some of its original paintwork scheme of light green, blue, and purple, matching the ceiling and wall treatment for the music room at 20 St James’s Square. The organ contains Snetzler’s great and swell organs which comprise of around 1,270 pipes and the instrument is still in playing order.