Dave,
My Clymer manual says that the A/C equipped cars used a 3 port vacuum valve. Under normal conditions, it routes the Carburetor vaccum to the distributor advance diaphragm. If the engine overheats while idling (because of the A/C load) , the valve routes Intake manifold vaccum to the Distributor ( using a higher vaccum to advance the timing) and increase idle speed , thereby cooling down the engine. I don't have a diagram to distinguish a 73 from a 72 , but let me know I'll be trying to scan the 72 diagram.
Pintosopher