in The Journal of the Playing Card, Vol. III. No. 1, August 1974:
"....Trumps amounting to fourteen are striking, for this is just the number surviving in the Colleoni trumps by Bonifacio Bembo, the artist who painted the Visconti set. About 1480, an artist working in the Ferrarese style supplied to this deck Strength, Temperance, Star, Moon, Sun and World cards. Is it not possible that in 1450 Bembo knew games with only fourteen trumps, but that 30 years later an artist compatible trumps to accomodate the new version of tarocco which became standard? Thus the later artist, contrary to all other current hypotheses, was not replacing, but adding the six Ferrarese trumps. By this reasoning, two more of his cards are missing."
The article is a little longer, but at this place Decker didn't explore his idea very far (ca. 1 1/2 pages).