The rear vent is actually the HIC, or Hot Idle Compensator. It was used to provide a controlled vacuum leak when underhood temperatures reached a determined point, or high enough point to cause the bi-etallic spring to open the valve and add some addtional air at idle speed.
I don't usually leave this system in place for high performance carburetor builds, but will on stone stock set-ups if the parts are still in good shape......Cliff