I'm sure someone has posted this information before, but I thought it might be useful to someone.
I recently had a couple of projects I had been pondering. One was replacing a free hub on a 7 speed hub and the other was fitting 130mm wheel into 126mm dropouts (I've read a lot on the different options). I bought a Shimano FH-M732 for the former that solved the latter. Since then I have bought a few other freehub bodies for other potential projects. I can't speak for modern freehub bodies, or the odd duck out there but this is what I stumbled upon.
Shimano made shallow and deep recessed hubs and that determines which freehub body will work. The one on the left is an MT-62 and the one on the right is an M732. They are both 7 speed UG/HG, but the recesses are not the same.
The following are Shimano freehub bodies that fit shallow and deep hubs. The ones (MT-62-7, 6402-8) on the left are for shallow and the right are deep (M732-7, 7700-9).
What I discovered was the M732 freehub body had too deep a flange and wouldn't work with the shallow hub, but it fit on my 7700 rear hub and converted it from a 9 speed to a 7 speed and fits into a 126mm wide dropouts. Not only that, I am able to use a threaded uniglide 14t with the hyperglide cassette giving me a 14-34 cassette. For years I have heard about swapping freehub bodies, but never realized that even though the same splines work, the depth of the hub to the freehub flange is the determining factor.
John