Conclusive evidence came from notes written in October 1503 in the margin of a book.

Discovered two years ago in the library's collection by manuscript expert Armin Schlechter, the notes were made by Florentine city official Agostino Vespucci, an acquaintance of Leonardo da Vinci, in an edition of letters by the Roman orator, Cicero.

In his annotations, Vespucci wrote that Leonardo was working on three paintings at the time, including a portrait of Lisa del Giocondo.