Linkbasic's CAB-MR42, is a 42U (1.866m) long, steel mounting rail that can be used in either the CAB-42U or CAB-42U1M. It is used to mount shelves, switches or other rack-mountable items/devices.
The rail is marked at every U-height in ascending order of U-height from both the bottom and top, so it can be used on either side of the cabinet and still have the correct marking in the correct place on the rail.