On the left is a bucked style of rivet, which is set with a rivet squeezer; it actually deforms into countersunk a countersunk hole on the back of the work to provide a flush finish on both sides.
These are aircraft rivets, used in applications where drag reduction is paramount, so no protrusions can be allowed.
Safe to say these pieces are securely fastened. I did a few more just to do it, and they all worked very well. I'm convinced.
That's for next weekend, though.