On the 67 CAP 4bbl that I have, the underside of the center , "shared" plugged port has no connection to any port. Another member has one that has threads in it like Mike's, mine is not drilled or tapped.
The break booster port is the one on the left IN the runner on both manifolds.
Yes, strange that it even exists. Like I mentioned, I have never seen anything connected to that shared port fitting. No clue what it is there for. Just speculating.