Good eye, mmfilesi; I hadn't noticed the lion.
You could just as well argue for a time after April 9, 1454, and the treaty of Lodi. However relations between Sforza and Venice were never good, even in peacetime.
In favor of the shield's having a Venetian lion is the book. Most lion heraldics didn't have them. Venice's sometimes did, because of the association to Mark and his gospel. However Venice always showed the book open. And Venice's lion didn't paw the book while standing on two legs. And if Venice was meant, you'd think there would be some indication of wings.
So I think the heraldic is just one made up for the card, with perhaps some vague legitimacy, if anyone challenged, from Muzio's lion. It might be there mainly for its symbolism. A closed book indicates counsel, according to one website (
http://www.americancollegeofheraldry.or ... mbols.html). A lion represents courage. It also was a typical heraldic of the German lords who elected the Emperor. E.g. it features prominently on the seal of the Golden Bull of 1356 (
http://wapedia.mobi/en/Golden_Bull_of_1356). Even the Gonzaga of Mantua had a lion heraldic, from the emperor in 1398 (
http://commons.wikimedia.org/wiki/File: ... zaga03.svg). Francesco very much wanted to be considered a lord of the Empire--not an elector, just an emperor-appointed duke.
Another possibility is that the King of Swords might be a representation of Francesco's son Galeazzo, the duke-to-be. He of course was a quasi-Visconti, through his mother; as a blood relative to a hereditary duke of the Empire, he might have had a better claim to dukeship than Francesco. So the shield might be a reminder to Galeazzo and his brothers to be courageous, to both give and seek counsel (notably from their parents), and to continue trying to be recognized as a legitimate duke of the Empire--a struggle that was eventually successful.