This has been a long but slow and very part-time search. Here (I hope) is a picture of the clip that is somewhere in Lake Winnisquam in NH. This is the clip I'm trying to find.
Look familiar to anyone?

Just lost a fender from my 1957 Sportsman. It went overboard (as I was distracted by a couple of granddaughters) with a clip attached to it, that clip fit into a slotted bracket inside the boat. I want to buy some of the clips because I know that this will happen again. Any guidance on where to look...