That's where a vacuum outlet port was. It probably fell off and got lost and someone put a screw in it. I am sure it leaks around the threads too. If it was mine, I would remove it and tap it for a set screw. Install the set screw with JB Weld on it and let it dry for a day or two.
UNLESS that's where the distributor vacuum advance went. Then you should replace the vacuum outlet and use it.
UNLESS that's where the distributor vacuum advance went. Then you should replace the vacuum outlet and use it.