That valve pictured above is the same set up used on 228 motors also. You would not have a down draft tube on your oil fill. Instead it is a small version of an oil filled air cleaner as that is where the air enters the crankcase. The valve connects to the intake at the base of the carb and draws air through. There was an excellent thread on oldgmctrucks.com that illustrates the complete set up.
Rich
