The present plaster bust is after the marble bust of William Murray, 1st Earl of Mansfield (d.1793) executed by Joseph Nollekens in 1779 and now at Kenwood House, London. Both the Kenwood bust and the Scone plaster are inscribed with Mansfield's motto 'Uni AEquus Virtuti' that translates as 'Faithful Unto Virtue Alone'. The motto is taken from Horace's Satires, and was used on Alexander Pope's bust by Roubiliac of 1740.
Family tradition dictates that the first Earl's nephew, David Murray, 7th Viscount Stormont purchased a cast of the bust from Nollekens. It is therefore possible that the present bust is the cast he bought.