odd crossover ports in early dual plane holley?

Picked up a holley dual plane from a member here to put a small 4bbl on my 318. I cant help but notice a small port under the carb flange. I have seen the notch before for what I assume is a pressure equalizer....but what's with the round hole? I vaguely remember seeing a hole in an iron intake somewhat like this years ago. Anyways...what is it? Thoughts? Thanks!