The article presents an illuminated Missal now in the Biblioteca Nacional of Madrid (ms. Res/6). The codex was written and illustrated for the Olivetan Monastery of San Michele in Bosco, Bologna, in about 1519–1520, where Giorgio Vasari worked twenty years later. The manuscript was profusely decorated by the workshop of Giovanni Battista Cavalletto, who painted three historiated initials, helped by a hand that the scholarship tends to identify with his son Scipione, who also executed three initials. The difference between the art of the father and son is a way to measure the passage between the second and the third Maniera, as theorized by Vasari, who probably turned the pages of manuscripts during his stay in San Michele in Bosco. The article also adds a new manuscript to the catalogue of Bartolomeo Bossi, a collaborator of Cavalletto, to whom the codex was previously attributed.