I have a Carter, not a Holley?

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.