Just back from two days driving the Skyline Drive...
Thanks to all for the feedback on the vacuum gauge connection.
Since I only got interested in this because of the Moss Motors video, and not because of any perceived problem with my car, and because I'm still not certain if the plug in my manifold will come out without a problem, I think I will leave well-enough alone and not fool around with it.
At least now, thanks the the feedback, I know what the plug is for and where I would attach a vacuum gauge if I ever needed to.