Sir Denis Mahon has kindly confirmed the attribution, and suggests that this is related to The Crucifixion with the Virgin Mary, the Magdalen and Saints John Evangelist and Prospero finished in 1625 for the Church of the Madonna della Ghiara at Reggio Emilia, L. Salerno, I Dipinti del Guercino, Rome, 1988, no 105, illustrated. A study for the picture which includes a Magdalen in a similar pose is in the Institut Néerlandais, Paris, The Italian Drawings of the Frits Lugt Collection, Paris, 1983, I, no.343, II, pl.395