What's this boaty bit called?

[Vid]

Please look at the photo - there's a nice round brass thingy that screws into the mast track to stope the sail sliders falling out of the wide part. To reef I have to unscrew it and take a slider or more out then screw it back it to secure the remainder. This frees up sufficient sail to reach the reefing horn.

Unfortunately it fell overboard last week so I have to go to the chandlers tomorrow to get a new one - what's it called?

Sensible suggestions welcomed...