This custom made tensioner pulley is used on the Rover K Series dry sump pump. It replaces the original component with an aluminum part which has been anodised black.
 
Normally the failure is the center bearing which causes damage to where it is pressed into the pulley. Alternative replacements are available in plastic, but this part enables you to fit an aluminum pulley.
 
Fitting is a direct replacement for the original, where the center bolt is simply removed to give access to swapping to the new part.