This is the Renishaw M6 Magnetic and Clamping Kit Base Plate #R-KC-MC4C-6.
The plate size is: 19mm x 750mm x 750mm. It is used with component C.
The M6 CMM Clamping kits can be used to hold brass, titanium, aluminum, plastic, and other nonferrous parts. A kit includes a base plate with M6 threaded holes 12.5 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.