Could be two things
TerryIf you are referring to the piece that holds the tack of the mainsail it's a tack hook. If it's the piece that keeps the sail from falling out of the track, it's often referred to as a track slide stop. You can find the parts at West Marine, Boat US or other chandlery. I included the url for the part at westmarine - but I don't know what size you need.ChrisPS Always buy two, especially if it's the part you need. They seem to jump into the water if you only have one.