We are grateful to Everett Fahy for pointing out that the prototype of this picture is the Saint John the Baptist by Andrea del Sarto, on loan to the Worcester Art Museum by the All Saints Church, Worcester, Massachusetts (see S.J. Freedberg, 'A recovered work of Andrea del Sarto with some notes on a Leonardesque connection', Burlington Magazine, CXXIV, no. 950, May 1982, pp. 281-288, fig. 22).
A rectangular version of the picture in a private collection, was attributed to Bacchiacca by Nikolenko (1966, fig. 65) in her monograph on the artist.