The starboard outboard side of the hull aft is the preferred HIN location for many manufacturers. On catamarans and pontoon boats the HIN must be affixed on the aft crossbeam within one foot of the starboard hull attachment.
Grumman is still in business and from comments in recent threads, they've been helpful on ID type questions. If you give them your HIN from paperwork, maybe they can help you find where they put it.