This is the MRS2 Back Spacer Kit  #A-6007-4008.

The MRS2 modular rack system is a versatile probe and stylus changing solution for Renishaw sensors.

MRS2 is available in a variety of leg and rail lengths to allow it to be configured to various applications. In instances where CMM working volume is at a premium, or there is a need to carry large numbers of probes and styli, additional rails can be added to MRS2 to create multi-tier racks.