Hi Nicolas. Looking at the photo, a couple fo things come to mind. The jammer fitted at the bottom to hold the boom down looks like it was once a Laser boom vang as I have the same set up on my Laser! I would suggest that the previous owner found this kept slipping when the halyard is under tension and he got bored of this happening and riveted the slider in place. I think you can drill out the rivet – I don’t see any down side to this, but you will need to come up with a better system to secure the gooseneck either against the halyard tension or gravity when not in use.