Arden,that should be attached to a PCV valve on the back of your carb,then routed to the vent tube set up that replaced old draft tubes on the block behind your distributor. This was the way it worked before the push in PVC valve that went into the valve cover starting in 68. 1967 and prior years used the push on vent/oil fill cap with this set up for engine breathing, 68 switched to a non vented screw on oil fill cap with the intake vent using a hose to the air cleaner,then the oil fill cap was located on the valve cover after that.