This is a replacement carriage for M2, M3 and Ultra One 3D printers, which can be used on either the X- or Y-axis rails.  Please note that rails and carriages are matched at the factory where they're produced, and carriages purchased independently of rails may not have the same fit as a paired rail and carriage.

This item ships attached to a plastic rail.  You will need to slide the carriage directly from the plastic rail to the metal rail on your printers to prevent dislodging of individual bearings.