While I am not sure of this Venture, there is a possibility that it was manufactured before MacGregor started using the HIN plate on the Transom. In the Venture 17, 21, 24 and others manufactured before about 1975, the hulls were identified with a number defined by holes in a stringer in the cabin. On my 1971 V21 it is on the stringer under the quarter berth on the right side of the cabin. You have to crawl into the left bunk and look under the cockpit to see it.
Based on the fact that you described it as a Venture of Newport, I would suspect it was manufactured pre HIN plate, as Venture became MacGregor about the time they switched to the plate.