Shimano Aero Technium XT / XTA Oscillating Slider 
 
This is a aftermarket manufactured part cloning the very popular discontinued original. These have been machined out of aero grade aluminium.
 
This part will fit;
 
Shimano Aero Technium XT 10,000
Shimano Aero Technium XTA 12,000
 
 
Part Number - RD8509