Davis Flat Large Sail Track Stop 
Flat/Large Sail Track Stops
Flat/Large: 0.75 x 1 x 0.125" (19 x 25 x 3mm)
When it's time to lower your sail you can rely on Davis' Flat Sail Track Stops to hold your sail and its slides in place on the mast and boom. Large knurled wheels are released with ease. Made of heavy-duty, black anodized aluminum and marine garde nickel plated brass. Flat stops come with lanyard holes.
Quick Guide (pdf)