In Portrait of a Musician, how is the Josquin des Prez of the Identity of the sitter elucidated?

In 1972 the Belgian musicologist Suzanne Clercx-Lejeune proposed the French singer and composer Josquin des Prez (c. 1450–55 – 1521) as the sitter. Josquin worked in the service of the Sforza family during the 1480s, concurrently with Leonardo. Clercx-Lejeune proposed that the words on the sheet music are "Cont" (an abbreviation of "Contratenor"), "Cantuz" (Cantus), and "A Z" (an abbreviation of "Altuz"), and that this meant that they were associated with a song with a descending melodic line, such as masses, motets and songs by Josquin. This theory has since been discredited; the notation is largely illegible and many composers were writing in this manner. As with Gaffurius, other portraits of Josquin do not show a resemblance to the sitter and, as a priest in his mid-thirties, he was unlikely to have been the painting's subject.
Suzanne Clercx-Lejeune masses sheet music Cantus Sforza family motet Josquin des Prez Contratenor Altuz