It does come out. You can lightly tap it upside down and it’ll come out usually/sometimes. Make sure you don’t lose the weight and discharge ball that are under it. I’ve held a 14mm socket over it (with carb upside down) and tapped down on to a table and lifted the carb off the socket between taps to see if the pieces came out. If so they get captured in the socket.
Mark recently had a reply detailing how to remove it if really stuck. (Username @65swb45)
For the rivet I took a pair of rivet squeezers to it until it increased diameter of shank bigger than carb hole. I put some gasket sealant under head then pounder it in with a rivet set. Carb is assembled on truck with no leaks.