The Tillotson Fuel Pump Plate for the High Volume or what is called a Double or Alky Stack is the middle plate of the three high volume pump plates.
Tillotson reengineered the Standard Volume Pump Stacks, added an additional plate, and redesigned the plates' layout to support more fuel volume. Therefore, the gaskets and diaphragms are also different.
It is common for carburetors to be converted to Methanol(alky) for racing applications. You will need a complete high volume pump stack and either a P6-MDG Gasket/Diaphragm Kit or a P8-MRK Rebuild Kit for HL and HW Carburetors or P7-MRK Rebuild Kit for HR Carburetors.