It is necessary to align hole in rear caliper cylinder with that circle at pad to be capable install caliper. It is possible see in picture that cylinder catch circle at pad.
http://www.shiftice.com/images/brake...IMG_5200-1.jpg

 

Took a dremel to grind it off. I saw one day it wasn't aligned with the piston and it's a pain in the *** to turn it.

 

When you screw the caliper piston back it, make sure the indents are indexed up/down/left/right. Then the nub on the back of the brake pad sits in the upward index spot on the piston.

 

I also ground mine off. My impression of it was, it was preventing the brake piston from turning as the pad wore down. My rear brakes developed play between the pad and piston.

 

It is for the emergency brake to operate correctly ob the rear brake pads. Make sure it sits in the correct position in the brake pads. It is set by rotating the piston with a special tool, or needle nose pliers.