My local Ace Hardware store has all different sizes of plastic and metal hole plugs - some are pretty small.
A plug like that might work!
Or a small piece of lead shaped into a ball the size of the hole, inserted in the hole and held on the back with a tap on the front from a small hammer, to form fit it to the hole.
I looked at several old carbs I have here and none of them have a hole in that spot, but that doesn't mean that the hole shouldn't be there. General Motors used several suppliers for their Rochester B Carbs and even though they were "B" carbs there was differences between them. All of them did have an air hole somewhere on the air horn. Most had two air holes, one on each side, or front and back.
I can see where it probably is supposed to be there, but you could try to plug it and see what happens. I think you will find it should be open - as a vent.
