If the hole has no freeze plug, someone has simply pulled out the tube. This isn't helping your motor. I'll post pics if nobody else does.
The tube has a bevel cut to the rear and the idea was based on cars being less stuck in gridlock and more often moving at highway speeds, than is common for many of us today.
Moving air past the tube's cut end creates a vacuum, drawing excess pressure and fumes out the crankcase. To avoid creating total suction inside there, the oil filler cap is vented, and stuffed with oil-soaked stainless shavings to trap incoming dust. That's why you had to wash your filler cap in clean fuel and shake it dry every 5000 miles (or less on dusty roads).
Later motors went to a rocker cover with two holes; the smaller one at the rear ran a PCV valve. This one-way valve was hooked to the intake manifold or bottom of the carb where suction was present. It similarly drew vapours and excess pressure out the engine.
You can go to a two-hole valve cover and plug the road draught hole, or restore the tube. Or - fit a PCV valve to an elbow off the original hole, and hook this to your manifold. That may not look so original but it will work better then the tube and doesn't require another valve cover.
Regards, Adam.
