The 'S' on the reverse of this dish is a signature for a painter thought to be Sforza di Marcantonio. The script on the reverse on the dish and the style of the painting on the obverse both correspond closely with other pieces thought to be by him. A group of pieces by this painter in the Herzog Anton Ulrich-Museum, Brunswick, are illustrated by Johanna Lessmann, Italienische Majolika (Brunswick, 1979), pp. 345-351. Before his arrival at Pesaro (by 1548), Sforza di Marcantonio worked at Urbino in collaboration with Xanto. For a discussion of their working realtionship, see John Mallet, Xanto, Pottery-painter, Poet, Man of the Renaissance Wallace Collection, London, Exhibition Catalogue (London, 2007), pp. 36 and 41-42.
The underside of this dish is illustrated at the end of the catalogue.