Measuring from the outside of the buttons demonstartes nothing, button sizes vary
Measure from center to center for a meaningful number.
The Castagnari is 21 mm (about 13/16ths) center to center and approx 183mm (about 7 1/8 th) center center of the outside buttons

And the reason the Shine Mouton and subsequent Mouton boxes use the "Hohner" scale (Euro for button spread)
yup... Shine Mouton had been known to use Hohner components. There is such a box locally.. Hohner aluminum slides. There is another with Hohner reeds.
Go Hohner. Again, glad to see they are being recognized and appreciated as an influence to Cajun box makers and players.

The buttons on both accordions have the same size, so the distance from center to center is also bigger by the Castagnari.
But it's remarkable that the distance on a much smaller accordion is bigger.

You're right Dwight, I also don't understand why I ever had bought those Italian accordion to play Cajun music. But it's not a bad instrument.

Ron putting aside the Italian box at least you were smart enough to purchase the finest LA box in the Jr. Martin brand.

Glenn, Is yours new ,or old and made be Shine Mouton..
Have you looked at the reeds and sliders?

What is the center to center scale button to button and outside button to outside button, center to center ?

Is the bellows size Hohner HA 114 ? or something else ?

It's an early 90's Mouton.. hefty.. VERY LOUD.. ( louder than any Martin I've owned and played ... and the Bb helps ).. amazing woodwork..( it is literally unplayed )..
but felt my hands had shrunk slightly and it was weightier than most ( maybe because of the wood.. "long pine ? ".. dark and light combo.. very pretty.

Certainly feels like the most expensively made Cajun accordion I've played. I owned a CAJUN brand once ( the JR builder not Doucet ).. done in Some Hawaiian wood with amazing details, rounded buttons..etc. Which was impressive as well.

As far as saying one brand is better.. I thing an individual accordion can be better or better for one or more players.. but I've seen substantial variation in most major brands.. ( not bad to good )..just different.. which I think accounts for tastes..

G.

Hve you looked at the reeds and the sliders
Are the sliders aluminum ?