Solid brass curtain rail runners for I-Beam curtain rails, old fashion curtain track runners. These are solid brass, you can check by using a magnet. Brass is not magnetic!
If you are happy with plastic roller runners for your I-Beam curtain track, then I would recommend these nodeko T-Rollers with Pleat Hook They are also considerably cheaper and incorporate a lockable curtain hook.
NOTE sold in quantities of 1, photo for illustration.