Dummett's conclusion is indeed suggestive. However it would not extend to the original design. Why the Bon family in particular? In any case, it would be worth seeing an image of that Venetian family's coat of arms, as a clue to what its images might have meant in the 15th century.
Nathaniel's quotations from Dummett prompted me to look again at what he says about the Ace of Coins (The Visconti-Sforza Tarot Cards
, p. 28):
The shield on the coin was originally red and silver; the silver has tarnished. It has been postulated that there was a design on the silver half of the shield, but it seems more likely that it was plain, like the shields on the caprisons of the horses ridden by the knights of batons and swords.
The Fournier Museum set includes an ace of coins but it is quite different: the single coin bears a design of a three-tiered castellated fortress.
It seems to me fairly clear that Dummett notwithstanding, there was a design on the silver part, both upper and lower, the upper being the Biscione. The lower may well be the "castellated fortress" of the Fournier card, perhaps the basis for my memory of castle ramparts: however my memory is of an image, not of words. It would be worthwhile seeing an image of the Fournier card; my cursory look on Google Images did not turn it up. It would be also worthwhile knowing if this "castellated fortress" appears elsewhere on any Visconti or Sforza card.
If in fact the left side of the card we have was red, that speaks in favor of the lower left having a red background. If so, the most logical choice for a design would be the white cross on red background, since the Biscione is there diagonally opposite, the device on the other banner of the CY Love card. If the others are Visconti heraldics, then probably so is the fourth. That speaks, however hypothetically, in favor of the white cross on red background on the Love card being that of Pavia rather than Savoy or someplace else. I do not recall that the Visconti ever appropriated that device of Savoy as their own.