Not all RCBS and Lyman dies these days are interchangable.
Somewhere in the dim past Lyman tightened up the specs on both their sizing dies and their lubricator/sizer. So my old Lyman 450 and my RCBS L/S can use any dies I've got. My new Lyman 4500, however, will not handle them all. Older sizing dies, either RCBS or Lyman, may have too large a diameter to fit in a new Lyman 4500.
Took me awhile to figure that out but Lyman did confirm the change.
My 450 was, I thought, just about worn out but, well, it will handle all the dies so I'm keeping it around.