I agree, they are historical -- art nouveau, just like you say -- and probably date back to times before any Cajuns knew what an accordion was!
I also agree that they are not 'pinstripes.'
If you look at ca. 1900 accordions, you see a lot of variations, and some that are much "finer" looking. Marcel Messervier Jr. has done a fine job in making his version of these finer decorations -- complete with naked nymphs!
Naked Nymphs in La Belle Province, oh Heresy, oh sacrilege .
But it's true that the clergy, doesn't have it's original stangle-hold over the general "population" anymore, thank God.
Do you have pictures of Messervier Junior's handywork?.
No, I don't have a picture of the Nymphs! Messervier Jr. was doing various designs, using a computer and making black decals that contrast with the light wood of the faceplates. The nymphs -- well, I don't know that they are really nymphs, it just fits the art nouveau theme, they might as well be called "naked ladies" -- were on a box that he made around 2002.
I've got some pics of Messervier boxes, but none that are close-up enough to see the patterns clearly. Anyone who wants to see one, BTW, can send me an email off-list.