In 1984 the Scottish National Portrait Gallery acquired two oil paintings signed by Pietro Fabris and dated to 1771. An old label on the frame indicates that the paintings were produced for William Hamilton, the British ambassador at Naples from 1764 to 1800, and the Viscount Kenneth Mackenzie, later Lord Fortrose, who spent time in Naples from 1769 to 1771.
While in the first picture, depicting a scene of fencing, a character sitting at the table has been identified as Niccolò Jommelli, there have been many doubts about the identity of the characters in the second picture, showing a chamber concert. They are now identified as Leopold and Wolfgang Amadeus Mozart, the first at the harpsichord and the second, waiting to play, in front of a triangular spinet.
This according to I Mozart e la Napoli di Hamilton: Due quadri di Fabris per Lord Fortrose by Domenico Antonio D’Alessandro (Napoli: Grimaldi & C. Editori, 2006)
Above, the painting in question (click to enlarge); Wolfgang and Leopold are on the left; Lord Fortrose in in the center, with his back turned, with Hamilton on his left and Gaetano Pugnani on his right, both playing violins. Fabris himself peers out at the viewer from the lower left, holding the painting-in-progress.
Below, Mozart’s symphony in G major, K. 74, which he completed during this visit to Naples.