All V-type dual plane intakes use a 'signal notch' in the plenum divider, always have and still do, of varying lengths and depths depending on design/manufacturer. The main purpose was the carb tending to lose it's vacuum signal between R & L cylinder banks intake valve openings causing erratic operation. Some folks may have needed to modify the notch back in the day of the 3v carb, but that definitely wasn't/isn't this opening's original purpose.
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