When did Leonardo da Vinci first use the letter or symbol "Y", for anything or reason?

I believe a vast majority of his art work has a "y" implicit in them i.e. Vitruvian Man there is a "y" symbol implicit in it. He also used the "y" (l'altro segnato) to catalog a series of painting books (Treatise on Painting) in which he drew about 950 individual pages in order to arrange them thematically.