I too have a 1989 NC 33. Where you have a rivet I have a pinhole. There is another pinhole higher up on my mizzen mast, below the gate for the sail slugs. An appropriately sized locking pin is tethered at the gooseneck. The locking pin is in the lower position when sailing with the mizzen but remains in the upper position when at dock or anchor or when motoring or sailing without the mizzen.