The Severne Carbon Race extension is Severne's top of the line extension for your SDM mast. The third roller on these extensions is the key to be able to use all rollers on sails with the 4 pulley blocks. These come on higher tension sails like the NCX, Moto, Hyper and Mach. Can easily be used with other models, like the Gator, that only have 3 rollers by simply skipping one of the extension rollers. 