The M91, M44, M55, V15 I all have identical generators. The only difference is the plastic mount the generators are glued to, and the styli.
The M55 and M44 are exactly identical in every way. The only differences are the styli. You can take any M44 body and slap on an N55E stylus and you have an M55E cartridge. Likewise, you can take an M55E body and slap on an M44-7 stylus and you have an M44-7 cartridge.
I use an M55E Swiss nude elliptical stylus in my M44-7 cartridge.
The M91 is the same cartridge. The difference is that Shure mounted the generator at an angle of 15 degrees in the plastic mount, instead of flat. This angle allows for a taller or thicker grip and a grip that is angled instead of straight like the N44, N55, VN15 grips:

The M55 and M44 are exactly identical in every way. The only differences are the styli. You can take any M44 body and slap on an N55E stylus and you have an M55E cartridge. Likewise, you can take an M55E body and slap on an M44-7 stylus and you have an M44-7 cartridge.
I use an M55E Swiss nude elliptical stylus in my M44-7 cartridge.
The M91 is the same cartridge. The difference is that Shure mounted the generator at an angle of 15 degrees in the plastic mount, instead of flat. This angle allows for a taller or thicker grip and a grip that is angled instead of straight like the N44, N55, VN15 grips:



