You didn't mention where you checked for the "non-hidden" HIN. Just in case you have missed it, on my 1980 Cherubini Hunter 36, the HIN is on exterior of the transome. Down low along the bottom edge. But can't remember whether port or starboard. Has yours weathered away? Or lost maybe due to damage repair or a sanding/painting project Mine is still plenty visible. I've been all over my boat through the years and haven't noticed another HIN. But also, I've never really looked.