This threaded knob is a direct replacement for OEM Part Numbers 55494020 & 554977. It fits many different years and models of Mastercraft boats including X-1, X-2, X-7, X-9, X-10, X-30, X-45, and X-Star . Typically, this knob is found near the base of the rear tower leg (behind the driver's seat). Before purchasing, please read below for details on how it is used for specific models and years of boats. If you are unclear, please send a message with a picture of where on the tower you are trying to replace the specific knob. Please include the model and year, and we will do our best to accommodate. For 2005-2006 MasterCraft X-Star: This year's tower has two black knobs at the base of the rear billet tower leg. Standing in the boat and looking at the two knobs, this is the complete replacement for the knob on your left-hand side. This knob must be COMPLETELY removed to fold down your tower. For 2005-2006 All MasterCraft X-Series: This knob is specifically used at the rear pivot point (behind the driver's seat) of the tower. There is only 1 knob securing the tower in this location. This knob is slightly loosened, and the rear half or “hoop “of the tower hinges on these knobs when you fold down your tower. For 2007-2011 All MasterCraft X-Series & X-Star Towers: This year's tower has two black knobs at the base of the rear billet tower leg. Standing in the boat and looking at the two knobs, this is the complete replacement for the knob on your left-hand side. This is the knob that you must COMPLETELY remove to fold down your tower.