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.
The rivets should be pressed (not hammered or pounded) together, such as between two boards. The long end goes in from the top.