The very original design of the Bxx engine just vented to atmosphere through the oil trap box on the side of the engine, not through the oil filler cap. The design change occurred when they implemented positive crankcase ventilation. B18 and B20 engines have used two versions of PCV. In one version air flow is in through the filler cap and out the oil trap and in the other the air flow is in through the the oil trap and out the filler cap. The B20E and F on 25 PSi's car used the in through the oil trap and out the oil filler cap as per Ron's diagram.
Ron's diagram shows the PCV getting filtered fresh air from just upstream of the throttle plate. That arrangement was a P1800 E/ES thing; but, apparently not all P1800 E/ES cars. On the B20E in my 1971 142 E filtered air comes from the bottom of the air filter hosing with a hose that loops over the engine to the oil trap. 25PSi's car looks to have that same arrangement. You can see the hose looping over the valve cover and the boss on the intake manifold where the port for the air inlet would be is undrilled.
Positive crankcase ventilation may help with emissions; but, it predates exhaust emission control in North America (1972). Its primary benefit is providing positive air flow through the crankcase to reduce build up of nasty stuff in the oil and as such is an engine life enhancing feature.
To Ron's question, my recollection is that the calibrated restriction on the B20E / F is machined in to the nipple fitting. To Rustinmotion's question about the additional air flow causing a lean combustion condition, The B20E / F is a manifold air pressure (MAP) controlled fuel injection system. The system monitors MAP and RPM and adjusts fuel accordingly. It does not care whether that air enters via the throttle plate or through any other path. The controller will works its magic to provide a correct fuel mix. What air flow through the PCV will do is alter idle speed. That calibrated restriction in the nipple fitting is critical to limiting PCV air flow so that you do not end up with a 2500 RPM idle speed.
I don't know for sure; but, it is possible that a previous owner used the plug in the PCV hose as a dodge to deal with high idle speed. The thermostatically controlled idle air valve on the B20E / F is a chronic problem area on aging D jet engines. If the idle air valve sticks in the full open or close to full open position you will end up with a high idle speed. Perhaps rather than fix the idle air valve a previous owner decided to reduce air flow into the intake by blocking off the PCV to reduce air flow and idle speed. If 25PSi reconnects the PCV and all of a sudden has an extremely high idle speed which cannot be adjusted down he may have an answer to the mystery plug.
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