IIRC, mine had one of each. Through-bolts to hold the cap on and a sheet metal screw through the insert. But this could easily have changed over the years and models. This is one of those things that I've had to "create access" to address, so replacing with all through-bolts now.
One of my inserts simply disappeared a year or so ago, so I'll be revisiting the issue. I used the vinyl stuff from Catalina Direct about ten or twelve years ago, and I suspect that it shrank enough to come loose. (But then why didn't the other side do so? Well, it's the side usually parked away from the sun.) IDK, remains a mystery.