I have a couple of original floorboards with the rectangular plates still attached. The semi-tubular rivet (with a large brazier type head) is installed from the bottom which then goes through the rectangular metal piece and is rolled over with a tubular riveting tool. I learned a trick from Tom Wesenberg for installing tubular rivets - put a drop or two of oil on the end to be peened. The oil allows the tool/rivet to easily slide and makes a very nice rolled over peen.
Rusty Nelson