This is a standard 84 Engineering Tool Arm used to mount any of their quick release pin system accessories including Contact Wheels and Rotating Platen, etc
 
Available for the Shop Mate 48 (SM-48) 2x48", Shop Master 72 (SM-72) 2x72" and Shop Master 2000 (SM-2000) 50x2000mm  Belt Grinder.
 
Key Features include:
Solid Aluminium bar 30mm x 30mm x ?mm (to pin centre) for the SM-48
Solid Aluminium bar 40mm x 40mm x 285mm (to pin centre) for the SM-72
Solid Aluminium bar 40mm x 40mm x 385mm (to pin centre) for the SM2000
 
Note: The SM2000 is suitable for use on the SM72 machines.