Well NOE makes a drop in expander plug for rifle and pistol cartridges that are like the Lyman M die expander plugs. You basically take out the factory expander plug in the Lee die and replace it with the NOE expander, You will have to make a spacer to take up the excess space because the Lee factory expander plug is much longer, some simply flip the factory expand over but I like to just use a couple spacers to give me some adjustment and keep the case up inside the die. I use them for all my cast bullet rifle and pistol cartridges that I use Lee dies for.I had asked on Cast Boolits about an "M" type expander for the Lee die and I guess there isn't anything.
The expanders have two numbers the first is the case mouth step which I like to be about .002" larger than the bullet dia. the second numbers is what the expander plug sizes the case or case neck to which I like to be around .002" under bullet dia. The expanders are designed to work with the Lee Universal Expander die but will also work in the Lee factory expander die.
Examples:
.357" would need a .359 x .355 expander.
.358" would need a .360 x .356 expander.
http://noebulletmolds.com/NV/index.php?cPath=565_89