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.