It's a diaphragm cover on the base plate. It's part #109 on this image - https://www.carburetorfactory.com/expvw33.html It also means you are missing the diaphragm spring, the spring seat and the diaphragm. I an taking a guess that it is part of the EGR control and you're probably got a decent vacuum leak from it not being installed. Just take a moment to admire that exploded view diagram of the 32-35DIDTA carb. The stuff of nightmares... Webers are a joy to work on compared to that migraine fuel Ditch the Mikuni along with the EGR trash, the feedback module and the 412 kms of vacuum hose that goes with it. Check the numerous Weber install threads on here to know what to buy to set it up and what to look for from a supplier and the actual carb.