Take a really close look at the stamped number.

Carter 2100s is a manual choke carburetor.

Carter 2101S is an automatic choke carburetor. The vacuum passage pictured in the bowl is the supply vacuum to operate the automatic choke on the 2101S. The airhorn on the 2101S has a matching passage to that on the bowl, that then goes into the automatic choke unit behind the piston. The vacuum bleeds from behind the piston to cause a vacuum inside the automatic choke housing, and draw hot air through the heat tube.

Should be able to simply plug the passageway and the carb will work fine.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
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The Carburetor Shop