^ Its actually very simple.

On the brake pads (infact on all of the pads) is a small notch that sticks out on the back face (not the friction side). When fitted normally this is supposed to slot inside one of 4 grooves in the brake piston.

I think a back to back test is called for some new rear pads fitted with the lug still intact, have a drive and see how they feel, maybe even a brake effort reading taken.
Then whip the same pads out, grind off the lug, do it all again and watch in amazement as your rear brakes rip the brake testing machine clean out of the ground

Well volunteered jimbo

I'm so all over it. Like gravy on mash