Speedy Streamline hook gliders for Streamline tracks.
Hook gliders were designed for situations where your have separate linings for you curtain. If you wanted to wash your curtains or linings, then you could remove either the curtains or linings separately to wash them. Therefore you could still have privacy by leaving one or the other in place.
Also referred to as glide hooks.
NOTE sold in quantities of 1, photo for illustration.