The Lyman "M" die body is the middle die in their 3 die pistol sets and has been available by itself probably for as long as they've been making 7/8x14 thread dies.
The die body is bored large, larger than any standard cartridge case, and does not touch the case anywhere, and all the die bodies are the same. The difference is the expander stem, which is a two step design, the lower part expands the resized case back to correct size and the upper "step" flares the case mouth. It is easily adjusted to only expand, or expand and flare. The expander stem is also a standard size and only the tips (which thread into the stem) are sized for the caliber desired. You can take an M die from a .44Mag set and swap the expander to .357 (or any other caliber) and it works just fine.
It was never a powder through die, and indeed was designed way before such things ever existed, or were even thought of.
If Uncle Nick says they have a powder through die under a different name, then they do. I've not bothered to keep up on those items since I gave up on progressive loaders about 20 years ago,,,,
