This is a Mounting frame to suit the 84 Engineering range of Radius Platens (with integral water jacket). Shown in the photos with the Radius Platen mounted (available separately) .
They include 2 wheels, solid adjustable tabs to suit the Radius Platens and a mounting pin for the 84 Engineering Quick Release tooling arm system.
These Radius Platens Mounting Frames are a direct fit to the Shop Mate SM48 (2x48"), Shop Master SM72 (2x72"), SM2000 (50x2000mm) and Gibson 72 Belt Grinders.
 
Related products;
Radius Platen
Radius Sanding Block