Brumbaugh studied and wrote extensively on Plato (e.g. "Plato's Mathematical
Imagination." Bloomington, 1954). It seems that he was investigating Plato
in Milan.
"In looking through the margins and diagrams [of the VMs] in the spring of
1972, I had a bit of luck. Some of the symbols were the same as those of an
'astrological' diagram which I had seen in Milan added on the back leaf of a
Plato manuscript. In the diagram, these designs represented numerals (modern
and archaic, 'Arabic' and Arabian), and this suggested, if the Voynich
cipher were a remote cousin, that the 'alphabet' of the latter could also be
numerical. Now, as it happened, this notion was confirmed by a set of
marginal 'doodles' or 'mystifications' on folio 66r." ("The Most Mysterious
Manuscript, The Voynich 'Roger Bacon' Cipher Manuscript", edited by Robert
S. Brumbaugh, Southern Illinois University Press, 1978, p.116).