I replaced those rubber top corner finishers last year, and I had the same question as you. When I asked in a forum, it seemed that there were several possibilities. One was that the hole was just "there"; another was that there was a rivet in the hole. People were saying that indeed the rubber was glued in place. In the case of our car (also a 1980 TR8), there was no hole in the flange behind the rubber. Based upon a recommendation from one of the posters, I used "Clear Gorilla Glue", and that seems to adhere well. For good measure, I also drilled a little hole into the flange and threaded in a tiny self-tapping screw.
Dave