What was the original use for the vacuum? I’d guess a power steering unit given it’s location but this car never had power steering. What sized bolt is that plug Mike? 3/4”?
The port is for power brakes- power steering has nothing to do with manifold vacuum. Size should be 3/8 NPT.
Agree with the above- someone probably misplaced the original plug and just used a spare sender to plug the hole.