The Tillotson External Pulse plate allows the carburetor to get vacuum from the plate directly rather than using the hole on the flange of the carburetor.

 Some Tillotson carburetors will come with this style plate from the factory when specific applications do not support pulsing off the flange. If you decide to make the change to this style setup be sure to plug the flange pulse hole. If both are vacuum ports are open this may cause irregularities in the pumping action of the carburetor. 

This external plate will often be listed for the HR Model carburetors since that is the most common application but this is a high volume, or what many call a double stack or Alky Plate stack, style pump plate. This external pulse plate works with any high volume(double stack, Alky) Pump plate carburetor or ones that have had the plates converted to this style plates. Check your carburetor and carburetor listing before ordering.