In addition to being able to accommodate a larger bell flare diameter, does this version of the case also have extra width in the body of the case like the Modelo F version does?

I don't know the answer specifically, but I'll tell you that when I contacted Marcus Bonna to ask which case I should buy for my Grennhoe Bach 50, they said the only one that would fit would be the MB-F because of the extra width it has. They didn't say that the XL doesn't have that, but they did say the MB-F is the only one that will fit my horn, for whatever that is worth.