RCBS handles should work on a Lyman mold, but may require a little shimming if they rattle. The setup may be a little 'nose-heavy', due to the extra length of RCBS handles, but it'll work.
Lyman mold handles will not work on RCBS molds without grinding the arms thinner (height and width). Actually, Lyman molds will not work with most other brands of molds - including most custom molds.
