Fourteen reads and no replies! I guess I stumped you all!
I think I solved the problem. The good news is that your handles are not in danger of rotting away in the sunshine. Here in southern Arizona we have many varieties of hungry critters, and it's been a very dry summer. I think my aft handles may have been an experiment in critter cuisine.
Incidentally, should you be wondering, those handles do not attach to something that locks them BENEATH the shell of the hull
. No, those woven hemp attachments simply go into a very thick hull in a hole about an inch to an inch and a half deep. The screws simply pierce the hemp and hold them there.