One part is listed to suit Advantage™ windows: a 38.7 x 22.9mm adjustable sliding door carriage and roller with a brass wheel. On a sliding window the carriage sits in the bottom rail under the full weight of the glass, and it is the wheel that fails — it wears flat or picks up enough grit to stop turning, and the sash then has to be shoved rather than slid. Brass is the harder wearing choice where the track has already been scored. The two figures in the title are what identify the part, so measure the old carriage both ways before ordering, as several sliding window carriages look alike at a glance and only the measurements separate them. Check also how it seats in the rail and which way the adjustment screw faces.