Used to prevent the mains'l slides from falling out of the sail track when the main is doused at the end of the day. When it's time to hoist the main again, just haul on the halyard and up it goes. No need to feed the slides into the track again. Fits the flat internal track extruded into the mast of your Catalina 27 or Catalina 30.
This solution is superior to Catalina's removable machine screw stop in that it is harder to lose. When it's necessary to remove the sail slides, just loosen the knurled nut by hand, slide the stop down past the entrance slot, and retighten. There's little chance of losing it. No need to find a pocket to put a machine screw in.
Includes large knurled nut. Plate which inserts into the track is 3/4" wide X 1" high X 1/8" thick.