The pic is upside down. I was not sure but I thought that used the choke piston in that housing (not an external choke vacum pull-off) which has vacum to the choke piston in the housing since it is the choke pull-off piston and it looks like maybe a plug or something is missing from the hole. Many older carbs before they used electric assist and electric chokes had the vacum that leaked past the piston go to a threaded fitting by the choke housing and that was hooked to a tube that ran down through the exh manifold and back to the carb airhorn as it pulled fresh air from the carb airhorn down through the exh manifold where it heated the air and then it was pulled into the choke housing to heat the choke coil as the eng heated up. But this carb looks to be set up with the full electric choke and does not use the heat line like they used years ago but with the choke piston in the housing it still has vacum in the housing to the choke piston to open the choke some when started. My guess without a better look is a plug should be in the hole. Ron