You are bidding on one New - OEM Mercury Optimax DFI Regulator Diaphragm.  This diaphragm fits both the air and fuel regulator on all of the Optimax 3.0L Model Engines (200 - 300 HP).  Additionally, it is the same diaphragm used for the fuel diaphragm on the Optimax 2.5L Model Engines (135 - 175 HP).  This is the latest version from Mercury and they are identifiable by their purple color.  The previous models were either green or brown in color.  This diaphragm is used in either regulator (fuel or air) on the port side fuel rail for the 3.0L engines, and for the fuel regulator in the port side rail for the 2.5L engines.  Most dealers tell customers they are not available separately and the entire fuel rail needs to be replaced.  That is no longer true, and they can be purchased individually without buying the entire fuel rail kit.