Hey Ronny_CTS
Its the vacuum diaghphram cover for the pump jet. It's pretty important for richening up the mixture on transition. there is a short j shaped vac hose that goes from it to ported vac on the carb body down below it. It is designed to turn itself on & squirt extra fuel when the vaccum signal drops. Basically the same thing as the pump jets, but vac operated rather than throttle position operated.