I think all the 66 and 67 manifolds had the shared port and some of those weren't drilled and tapped. I don't believe I have ever seen anything plugged into that shared port though
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.