Professor George Knox has kindly pointed out that this is a study for the head and hands of the Apostle on the left side of the Assumption of the Virgin in the chapel of the Residenz at Würzburg of 1751-2, A. Morassi, The Complete Catalogue of the Paintings of G.B. Tiepolo, London, 1962, fig. 107. Other studies for this altarpiece are at Leningrad, the Correr Museum, Venice and elsewhere, G. Knox, Giambattista and Domenico Tiepolo, A Study and Catalogue Raisonné of the Chalk Drawings, Oxford, 1980, p. 336 (with a list of all the drawings for the painting)