This is the Renishaw M8 Magnetic and Clamping Kit Base Plate #R-KC-MC4B-8.
The plate size is: 19mm x 750mm x 750mm. It is used with component B.
The M8 CMM Clamping kits can be used to hold steel, aluminum, plastic, and other nonferrous parts. A kit includes a base plate with M8 threaded holes 15 mm on center (choose from plate sizes) and a set of components (A, B, or C) along with plate clamps for mounting to the CMM.