What's this mast part?

[Tuxie]

I finally got my boats mast up after fixing many other problems and when I got the main sail up I noticed there's small part missing which helps that the mainsail riders stay in the groove. Is this a custom part or something commonly used? What does this part normally look in the masts?

[Calif.Ted]

Usually called a mast gate, sometimes a plate that slides to the side, sometimes spring loaded. I just pulled up this page on google that seems to have a good explanation.
Mast Gate

[Tuxie]

Thanks! Just what I was looking for. Definately saved me few hours of googling (:

[Calif.Ted]

You could also use a simple "track stop", a sort of clamp you place in the mainsail track above the opening so your sail slide (riders) won't fall back out.
Sailrite - Fabric, Canvas, and Sewing Machines Since 1969
A gate is a little more convenient and secure perhaps but will be a little trickier to fit.