This is a sliding pillow block that fits on 12mm tracks. When you purchase our MP-LR12-400, that part comes with a MGN12H sliding block. The difference between the two are the block sizes that result in MGN12H and MGN12C. This part is excellent for adding as a replacement unit or purchasing individually to add to an existing track for more pillow blocks.
Highly recommend for replacing the X and Y axis of any 3D printers.