The attribution to Guillibaud was kindly suggested by Neil Jeffares. Guillibaud, son of a painter from Grenoble, was based in Geneva but seems to have travelled widely.
The sitter of the present portrait has not been identified with certainty. He is holding a musical score which appears to be by Antonio Sacchini (Florence 1730-1786 Paris), suggesting the composer as a possibility. Sacchini studied at the Conservatorio in Naples, and is remembered particularly as a composer of serious opera. During his lifetime his comic operas and light arias were also popular, and he was in much demand as a singing teacher. His works were performed across Italy, and in 1770 he travelled to Munich and Stuttgart, moving to London in 1772 and then to France in 1781, when he was presented to the Queen Marie-Antoinette at Versailles.
The sitter of the present portrait may be one of Sacchini's private patrons, the quartered posthorns on the coat of arms suggesting an affiliation with the House of Nassau-Orange.