Bill's TF link called the valve: Positive Ventilation Valve, I said PVV. Yes, the system is called Positive Crankcase Ventilation. Bill's pic is the V8 version of RPO 242. First available in 1961 in V8's and 6's.
Sorry for the confusion.
Twist on, vented, valve cover, oil filler cap for 1961 235 cu in as opposed to push on that fits on the front fill tube on V8's, which I find plenty of. I haven't looked very hard yet, just at FLAPS. I will check the vendors.
