Duels of the Plainswalkers is missing from the editions list and I can't list a few cards properly because of it (I'll have to use closest matching alternate)
It uses the same M symbol that a couple of other editions use.
Example cards affected:
Wurm's Tooth http://magiccards.info/dpa/en/97.html
Demon's Horn http://magiccards.info/dpa/en/91.html
Card list on MagicCards.info
http://magiccards.info/dpa/en.html