I take it the H in HIN stands for hull? That number is obvious and is the one I mentioned I think may have been re-riveted from another boat. Is it stamped anywhere else I meant?
Take your first three numbers of your HIN and put it in the MIC box, at least you'll know if it belongs on that model http://www.uscgboating.org/recalls/mic_database.htm
As for a duplicate number ( depending on the year ) there should be one stamped as a mirror image to the one outside on the transom. This one will be inside the hull.