I have the same thing on a Navarro "Universal" dual manifold. It has to be the vacuum port for the windshield wipers, because at least on mine, there is no other place to hook them up. If there is no other vacuum port on yours, that has to be what it was for, as Edmund's main market when these were new, was improving the performance on the "Average Joe"'s car; not racers or hot rodders.
I will agree however, that is not a particularly attractive location.