Mercury Part #: 8M6002988 :

8M6002988:

  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 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.

The Air/Fuel diaphragm for sale is not the tracker diaphragm that is installed in the starboard fuel rail on either the 2.5L or 3.0L Optimax Engine.  

This diaphragm is the flat top.

 Inside with steel balls