All I know is on the 21 they used aluminum rivets and then taped the cap to the sides. The size is like a 10/24 bolt. You should only have enough of the rivet sticking thru the combined material and washer to get a mushroom when you pull it. Anything more than that is actually a determent.
I think the reason they use rivets rather than bolts is a two fold. First faster and second it has a consistent pressure on each one applied.
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