Below are the vacuum circuits for a '66-67, which is essentially the same as your '65 as to the vacuum lines. The diaphragm in the engine compartment either goes full travel for "outside" air or partial travel for "inside" air. Note that "inside" still has some outside air (I guess because everybody smoked back then?). "Off" is no outside airflow.
Notice how each diagram also shows how the vacuum switch routes the vacuum.
Also notice that "outside" has vacuum to both ports. "Inside" has vacuum to only the offset port.
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