The PVC is connected to the front elbow that would be facing the opposite direction??
When its in use in a closed system its routed to the drivers side breather with no valve as we have discussed in other threads.
http://www.concoursmustang.com/forum/index.php?topic=12943.msg79337#msg79337
Don't disagree with the PVC system was a requirement but not sure why you bring that up since it has no connection related or physically is connected to the air cleaner.
In reality, the PCV system requires air to be drawn into the engine to eventually be drawn into the carburetor. It is called a "closed emission system". That air is drawn from the front tube, bent to the left (thank you for the picture) into the left valve cover fitting (the one used to add oil) crossing over to the right hand side thru ports in the intake manifold (usually plugged with crud) where it gets drawn into the PCV hose into the carburetor. There is a check valve on the right PCV hose (Ford base part number 6A666). Older PCV systems (like 65) used just a breather on the left side. An additional part is that gasses that built up in the engine during a low vacuum time frame are vented into the air cleaner thru that same left hose.
Just to clarify for terms of reference:
PCV: Positive crankcase ventilation. (Correct term used in context of this discussion).
PVC: Polyvinyl chloride. (The rigid form of PVC is commonly used in construction for pipe).
Dyslectic am sometimes I. Yrros.
What plug type / material / size would have been used for the rear nipple plug on a non-smog car?
Rubber. I got another one that had spring type "clamp lines" on it. I don't remember if there was a Ford part number on it.
Would love to see something that would prove something different
Personal experience. When I came (back) to California in 1967 with a 66 "A" code Fastback (Metuchen built, DSO 11) that had an aftermarket HP type air cleaner (it was cool), I had to get a new Ford air cleaner (Tuban Ford, Mountain View), left hand valve cover vent and hose, to pass California emission (smog) requirements in order to register a "foreign" car. That air cleaner had a plugged port on the rear. That's all that was available. After all these years, you remember the DMV and its regulations.
(Note: After the single use of passing a visual DMV inspection, that air cleaner resided in my storage locker being replaced with the HP type. It was put back on the car when I traded it in.)
Jim