Karl's always trying to show everyone his riv nuts

Seriously, its a neat setup. They are rivets that have threads in the opening. So after they are installed, you have multiple places where the old rivets were to screw your track to the beam.

Makes it easy to install or remove the jib track for those of you who switch back and forth from sloop to uni.

Oh, just realized you are talking about the main on the rear beam.