The argument in favor of Lorenzo could be that he is said to have taken in charge the family affairs as soon as 15 years old (1464?) and officialy at his twenty (1469).
http://www.histoireeurope.fr/RechercheL ... +M%E9dicis
Yet Pietro remains the Head of the Family even sick until december 1469.
The change of Palles with the Fleurs de Lys integrated given by Louis XI to Pietro de Medicis and his "descendants" .occurs only after May 1465.As the Charioter of the psCh VI has no Fleur de Lys, it has to be, probably, before this datation.
Coming back to Pietro .
You wrote :
"As long Cosimo was alive, he was the head of the family. I never read anything different. Naturally Cosimo had delegated a lot of things to his sons. Piero had the commission to care for art and literature, as far I remember, just following his interests. This doesn't mean, that he changed the palle, a global family symbol. "
Well, we know that Pietro was named Gonfalonier of Florence in 1461, just as his father and gran fathers had been before.
This is more than arts and litterarure..
"Pierre reste longtemps dans l’ombre de son père, même s’il occupe en 1461 la fonction de gonfalonier de justice.
Lorsque Cosme meurt le 1er août 1464, Pierre lui succède à la tête de la famille (et du parti) qui domine Florence, à la fois par le contrôle des institutions et par une politique d‘alliance avec d‘autres grandes familles florentines"
https://fr.wikipedia.org/wiki/Pierre_Ie ... C3%A9dicis
What did it consist as charge?
"À Florence, le Gonfalonnier de justice est le chef du gouvernement à l'époque de la République.
Dans cette ville, le gonfalonnier était l'un des neuf citoyens sélectionnés tous les deux mois pour former le gouvernement de la seigneurie de Florence. Il avait la charge de gérer la justice et il était gardien de l'étendard de la ville qui était porté accroché à une hampe croisée. Pour le distinguer de ses huit compagnons, sa veste était bordée d'hermine et décorée d'étoiles dorées. Le même titre est aussi utilisé à Lucques et Sienne."
He was also ambassador to Sforza in 1450 and to Venice in 1454
https://books.google.fr/books?id=rKFWAA ... 61&f=false
Clearly more than hobbies in arts and litterature...
Did this legitimate him to have arms before his father's death? The stained glass of the Oculus of Pierro
found by Steve is dated circa 1460 - 1462...
"As a ruler of Florence Piero di Cosimo de’Medici was in regular touch with the Mamluk Sultanate - he was also a collector of Mamluk metalware and glassware from Cairo and Damascus and it is possible that they provided the inspiration for this design for the stained glass oculus for the Oratory of Piero di Cosimo de’Medici in the Church of the Most Holy Annunciation in Florence - c1460/62
The Arabic Nashki/Thuluth script surrounding Piero's coat of arms, a script reserved for the Mamluk ruling elite, is of laudatory epiteths (his excellency - the Lord - the industrious - the wise - the royal) as if Piero is almost seeking to portray himself as some sort of Sultan or Emir of Florence!
The central motif is of course the familiar seven palle, the ring and feathers, and the motto 'semper' -"
This corresponds to the psChVI Charioter first rows of 7 Palles
As for the rows 8 Palles underneith the first row of 7 Palles, I think they may be an allusion to his father and gran father's arms - even if Cosme could also have a 7 Palles Blason ... Kind of legitimacy of his right from lineage Gran Father / Father/ Son ... (Btw also 3 Gonfaloniers of Florence)
Mikeh? Phaeded? Someone else?