The factory race option inlet manifold mounted two solex carbs, much like Webbers, & obviously the manifold had the oval ports, but was drilled with the earlier, center stud, manifold stud pattern. I do not know if any other manifold was ever offered with this stud pattern & port configuration. GX heads came with the extra holes, just in case one wished to fit this manifold.
Do the A14/15 heads have this extra hole? The GX also had an extra water outlet hole under the rear pair of inlet ports. A14/15's seem to have only 1 at this location.