The fact remains that almost any musette tuned box supplied to the UK will probably have Scottish musette tuning in one form or another, unless otherwise specified, as that is what the manufacturers believe we are looking for. For example, the Italian firm Fisitalia offer their accordions with a choice of 6 basic tunings, and they are Swing, American, German, Italian, French, and Scottish. On the basis that there is no "English" musette tuning, then they simply assume that UK players will demand Scottish tuning, and it really is as simple as that. Those tunings apply regardless of whether there are two (MM) or three (MMM) reeds, and that causes an element of confusion. In instruments which only have two flute reeds it is relatively easy to work out their advertised tunings, as if we are talking in "cents" above normal pitch, swing starts about 2 cents and Scottish is somewhere about 23.5. The next "strong" tuning down the list is French at 20 cents.
Now, most French boxes only have 3 voices, and these are usually arranged LMM (bassoon and two flutes). The various styles of music they play tend to require that the high flute is a bit tame, and it will typically be less than 10 cents. Away from the three voice musette, which still has a following in France, "americain" has been the accepted standard French tuning for the majority of boxes since about 1950, and anything above 10 cents takes the tuning out of that category to "moderne". However, even that category does not go above 12 cents, and the reason for that is if you select what they call "plein jeu" or the master to we anglophones, then that register is going to sound pretty dire (to them) with the straight tuned bassoon and flute reeds being compromised by the sharp flute. It's just not their sound, is the best way to describe it.
French "musette pur" accordions are still made in the three voice version, and are most commonly found as MMM instruments. They tune them exactly the same as Scottish musette, i.e. one flat, one straight, and one sharp, but obviously the pitch difference tends to be narrower than Scottish tuned boxes, as is their preference. The fact that you can get very strong French musette and lighter Scottish musette gives the impression that the two are interchangeable, but I'm sure you guys are in a better position than I am to come to any conclusions on that score. All I can say is that Yvette Horner and Emile Vacher would possibly sound French on a Scottish tuned box with regard to technique, but to ears like mine I would probably detect something "foreign". There have always been French players who play pasos, South American, and everything else on full three voice musette, and Andre Verchuren made a fortune out of it. However, most of the big name professional players did not use three voice musette at all, and some of them dropped it out at some point in their careers. Others would make albums with a three voice "musette pur" box, then switch back and forward between that and "americain". At first I wouldn't listen to anything but "musette pur", but I eventually began to appreciate the "other" French tunings. With regard to the popularity of "musette pur" over "americain" it was probably a close run thing at times, and musette pur does seem to be making a comeback in France, although you wouldn't really think that when almost everything French we hear these days in the UK has connections to Galliano or Ludovic Beier, with not a three voice tune in any of their styles.
The mainstay of three voice musette has always been in the very north of France, where players like Verchuren, Aimable, and Edouard Duleu hailed from. Guys like Gus Viseur, Tony Murena, Marcel Azzola, Andre Astier, Joss Baselli, and Jo Privat literally "swung" a lot of players away from the musette pur style. Even Maurice Larcange, who made most of his money recording "musette pur" eventually changed over to "americain " and "moderne"
With regard to boxes which only have two voices, and which are common on the Continent, any tuning from swing to strong(ish) musette is possible. The fact that there is no register combination available other than two voice or single flute means the sharpness of the high flute is not so critical, and you'll often get such French instruments where the high reeds are "up" to maybe 16. They'll not usually get much higher than that as there is no third bank of flat tuned reeds to balance things out. I have such an instrument that sits between 14 and 15 cents, and wouldn't really want it any sharper than that. If Fisitalia supply boxes which have two flute reeds to Scottish tuning. i.e. one reed true pitch, and the other at 23.5 cents, then I don't think I'd want to be anywhere near it when it was being played! I suspect they only supply Scottish and the 20 cent version of French tuning on three voice musette.
When you actually bring those flattened reeds into play then that's where I have to stay in the league I'm in, as I have no real idea what little tricks the tuners do to get variations in Scottish, Irish, or any other three voice musette tuning. Jimmy Clinkscale imported a few instruments from Cavagnolo, Crucianelli, Maugein, and Piermaria in the late 70s/early 80s, and the majority of those had very strong three voice musette tuning, nearer Scottish than French. The very few ones (LMM) which never had musette tuning were referred to as "Paris tuned" by staff in the shop, with the caveat that you were about to buy something that "didn't sound right" to Scottish ears, and you would have a hell of a job getting rid of. However, even those boxes with only two flutes were tuned pretty wide, possibly to Jimmy's own specification. I don't think there were ever more than one or two in the Melrose shop at any given time.
In my experience most Scottish boxes usually come as LMMM with the musette tuning taking preference over everything else, even if that throws some of the single reed options way too sharp to be used with the basses. Sorry if this post had almost nothing to do with Scottish tuning, but I couldn't try to explain it any other way.