Rubber joint provides shock absorption in choppy conditions. This unique design is very low to the deck with four finger ribs for easy adjustment. 1-Bolt Rubber Mast Base US Cup 1-Bolt Twist-On Mast Base Universal Complete US Base Cup Rubber joint provides excellent performance Very low to the deck Four finger ribs for easy adjustment US Base Cup attachment to extension ONLY AVAILABLE AS 1-BOLT US BASE AT THIS TIME