With a letter from Carlo Pedretti to Houthakker, dated March 5, 1972, regarding the authorship and ascribing the drawings to Martini, indicating that, "One of them may even be interpreted as the record of a device that Francesco di Gorgio had sent to Leonardo . . . there are drawings by Leonardo that reproduce the same instrument.". According to Brown, Martini was, "Active as an architect and architectural theorist, engineer, sculptor, and painter, [he] has rightly been called the 'Leonardo da Vinci of Siena.' As demonstrated by a series of exhibitions in the 1980s and 1990s devoted to him and his contemporaries, he was the most significant artistic figure in the city during the second half of the 15th century. Francesco was keenly responsive to the work of other artists--including Donatello and the North Italian miniaturists Liberale da Verona and Girolamo da Cremona, all three of whom worked in Siena--as well as Pollaiuolo and Verrocchio, whose productions he came to know in Florence. As a painter, he seems to have been trained by the painter/sculptor Vecchietta or, according to a more recent hypothesis, Sano di Pietro. Francesco is known to have shared a workshop with yet another painter/sculptor, Neroccio de' Landi, from the late 1460s to 1475, when their partnership was dissolved. Not long afterward he left Siena to work as sculptor, architect and military engineer, for Duke Federigo da Montefeltro in Urbino. His services were also in demand at the Sforza court in Milan, where he must have encountered Leonardo, who owned and annotated one of his manuscripts (now in the Biblioteca Laurenziana, Florence). Francesco's only signed painting is the Nativity with Saints of 1475, from the Olivetan monastery of Porta Tufi, now in the Pinacoteca Nazionale, Siena" (see Brown, Italian Paintings of the Fifteenth Century, Washington, D.C., 2003)