The Carter WA-1, 1 barrel carburetor can have one check ball in some cases and in other cases, 2 check balls. Most WA-1 will need both check balls.
Both types will need a check ball in the main discharge tube. When you have two check balls in the carburetor kit, install the bigger of the two in the main discharge. When required and for most WA-1 carburetors place the smaller (and sometimes aluminum) in the accelerator pump well.