The die body used for a custom plug is normally of little importance. When I make M-dies for fellow shooters, I use whatever old die they give me or I have in my scrap die box. The .218 Bee die set I just sold to a friend of CW's used the full length resizing die as the M-die. The slug had a 1/8" (maybe it was 3/16" stem that slipped into the collet for the original decapping pin Others I've made were made from a 9/16" SAE fine bolt because the die companies have a love affair with that thread. Other have been made from 1/2-20 bolt. The die body, as long as it will accommodate the M-die slug function, is just a method to attach the slug to the press. The expander slug in this thread is just another type of slug.