Brahma clamp rail to fiberglass fastening

I inherited a disassembled old brahma shell with a '86 Custom Cab that I picked up last year. I've just started to refinish it and had some questions about how the aluminum clamp rail was originally attached to the fiberglass. I've cruised around town trying to look into other people's canopies to figure it out but I'm hoping that some of you can confirm how these old canopies were set up. All the ones I've seen have had hex head screws on the outside of the clamp rail and some sort of square aluminum tubing on the inside. My thought is that those are self drilling/tapping screws that go through the rail and fiberglass and into the aluminum tube? But based on the looks of mine, I would have have guessed that it was originally riveted.

Can anyone confirm how these old brahma caps were assembled? I just want to make sure it is good and road worthy; letting go of a cap is not something i want to mess with.

Some pics for fun.