Cool, glad you solved the mystery! And... no more list :thup
I've just recently removed about 20 of the rub rail fasteners on my boat. Don't know if yours is the same, but on my 22 the rail is riveted with aluminum rivets, and they corroded amidships where the low point of the rail is, then the heads just popped off (and hence the rail gapped away from the boat in that area on both sides).
I removed the black rubber trim piece for a distance on each side, and most of the affected rivet caps were just lying in the rail base. On the inside, each rivet is terminated in a "blob" of something like 5200 that is gelcoated over. After reading a tutorial here by someone else who did a rubrail repair, I used a punch to firmly tap the rivets from outside the boat toward the inside, then took a cutter that is normally used for cutting fabric prior to setting grommets and held it over the (now visible) "pimple" in the 5200 on the inside. Twisting it caused a neat circle of the gelcoat/5200 to come off and then I could just pull the rivet shank into the boat, leaving a clear hole where the original failed rivet had been.
I decided to use machine screws to re-fasten the rail, and then debated between aluminum (similar metal so so issues that way) or stainless (stronger but dissimilar metal). I haven't done this part yet, but think I'm probably going to go with the stainless with insulating washers. I may cut a few small slits in the bottom of the rail so water does not pool there and "soak" the metals. I'll probably pump some 3M 4000 (from a caulk gun) into the holes from the outside prior to fastening. Some of that should ooze into the area behind the rail as well as under/around the fastener. Then I'll put the rubber strip back over the outside. (I don't like to caulk around the edges of things, so won't caulk along the outside of the rail.) On the inside I'll use acorn nuts where they will show, and regular nuts where they won't (most of mine are under the galley counter).
Nice dramatic "un-listing" after you drained your flotation foam!
Sunbeam