Good used axle vent nipple or tube. This vent tube screws into the top of the rear axle and is a breather / vent to equalize the pressure inside the axle tube. Fits both 8 inch and 9 inch rear axles. Fits Mercury Cougar and Ford Mustang 1967-73.
A (5/16" inside diameter) rubber hose about 18-1/4 inches long attaches to the nipple. Has a very tiny diameter hole inside, about 0.062". Needs to be kept clear to work properly. Also see the reproduction version from AMK. Concours hose kit with hardware also available.
Ford number is C5ZZ-4022-B, but AMK and others list the fictional part number of C8ZW-4338-A. Ford chassis assembly manuals, indicate that this is part number 4022.