I guess I am confused: I thought the Lyman "M" die was for rifle calibers... as I understand it and in my experiences, expanding the neck of cases works best in rifle calibers, and is unnecessary in handguns. As a matter of fact, brass/case tension in handgun calibers is what keeps a bullet from moving under recoil, so the case has to be very tight against the projectile. The "M" die would absolutely eliminate the capability of doing this.
I have shot thousands of handgun rounds loaded with conventional handgun caliber dies and never used a "neck" expander for a single case mouth, primarily because handgun cartridges typically have no true "neck". I have used the standard "expander die" to flare the mouth of the case prior to seating a projectile, but that is it.
And as for the photo above, the seating plug in the seater die will square the projectile during the seating process if the proper profile for the seater plug is selected.
Finally, I have to believe that the die manufacturers would offer an expanding die for straight wall handgun cases rather than just a mouth flaring tool if there was any benefit to using one for handgun rounds. But I may be completely out to lunch...