So I took an "M" die apart and found that the expander simply screws into a larger plug which screws into the die body. When we buy them, though, we have to buy the entire die set up. Does anyone sell just the threaded expander section? I see where Sinclair sells a body that uses interchangeable mandrels which are held in with a set screw. Sinclairs dies, though, stop at 8mm while Lyman goes up into .45 or bigger. What to do???

You can get the expander plugs from Lyman or Western Bullet.
http://missoula.bigsky.net/western/index.htm

You can do just about anything skinnier than a .50-90 Sharps or .348 Winchester with two M-dies and an assortment of expander plugs. (I think you could do everything with a .50-90 M-die and a pistol die, but I haven't checked that out.)

There's a long and a short die body, and a long and short top screw. Long cartridges like the .30-06 use a long body and short top screw. Short pistol cartridges like the .45 ACP use the short body and long top screw. Swap topscrews for intermediate length cartridges like the .35 Remington.

Bye
Jack

Thanks, Jack. I just visited their web site but could find only the entire M die for sale so I e-mailed them for further information.

Follow-up, re: Jack's referral to Western. They have everything in stock that I wanted (7 calibers) for $5.00 each for just the expander plug. Great service. Thanks for the help.