The rivets that hold the clamp wings in place. These are under warranty. If yours have broken, it is because too much torque has been placed on the wings either from 1) installing it too low so it is rubbing/hitting the top tube of your bike or 2) installed at the wrong angle. Please fix this issue once you replace the rivet to avoid it happening again.
If you have a press, it is best to press the rivet ends together. However, you may also use a rubber mallet to pound the smaller end into the longer end. The long end inserts in the top is in this video.