On pg. 28, fn. 21 I note that the phonologically distinct use of and
is found first in the earliest Spanish printed books. According to Kukenheim 1932:32, 36 it was specifically the great Spanish grammarian Antonio de Nebrija who was the first to suggest the distinction in his Gramática de la lengua castellana in 1492:
The distinct use of u and v was suggested by the Italian printer Trissino in a 1524 letter to Pope Clement VII.
See Kukenheim, L. 1932. Contributions à l'histoire de la grammaire italienne, espagnole et française à l'époque de la renaissance. Amsterdam: Noord-hollandsche uitgevers-maatschappij.
Must, Gustav. 1965. The Symbols for /i/ and /j/ in German Orthography: An Historical Survey. Modern Language Notes. 80.5:584–95.