They're riveted, but not pop-riveted. More of a brad style rivet, with a round top. Below are some pics of what it looks like.
I have what the factory called a blow-out clip "field fix package". It's for a 73-75 A-body, but they are similar to f-body as far as having frameless door glass and a blow-out clip. What they did was pull the weatherstrip to the side enough to be able to drill out the old clip rivets and remove the old clip. Then the kit includes 2 small screws to screw the new clip in place. The screws are #8 size, 1/4" long, with a 1/4" diameter pan head.
