I do not know about Norton grapes specifically, as I only grow vinifera, no aestivalis. So do not know how much the below applies.
The rule of thumb for vinifera to cut off any clusters forming later in the year. They will not ripen, but the plant will spend energy trying to ripen them, which means less energy spent on ripening the earlier spring clusters (resulting in under ripe grapes all over the plant). And the extra energy the plant put into trying to ripen the never to ripen late clusters will be better placed into late season root development for next years growth.