The Mars line. M series are birch shells. Sometimes called M Birch. M Pro are maple shells. New Mars kits are still all birch. I think the Pro M grew into the Meridian line (again with Meridian Birch and Meridian Maple), which grew into the Armory line (and is now hybrid maple/birch shell).Now I'm on to a Mapex M Series in transparent orange or transparent honey amber. Same drum sizes, I think.