The spigot on the back of the oil tank should be connected to the spigot on the crankcase. On the later tanks, this not only drains any condensed oil from the oil/air separator in the oil tank, it also vents any air from the tank, which is why the spigot is larger than the early tanks with separate vent hoses.
The main crankcase vent is the spigot in the cam cover, which was connected to the "collection" tank, and can be left open to atmosphere, as long as any oil from blow-by doesn't drip in front of the rear tire.