S3 Rear Brakes - Serrated shim between piston and pad? Is this normal?

[Chris_L@NZ]

Hello, I'm replacing rear rotors/pads on 2009 S3 8PA and when I removed the floating portion of the caliper and pads, there was this serrated shim or washer thing between the piston face and back of the pad. I've never seen this on a brake system before. It's not even same diameter as the piston but perfectly centred so seems like it was there intentionally - see pics.



New parts are Pagid and while they came with shims, bolts, there def isn't one of these things included. Can anyone fill me in please? Do I discard it? or find a way to somehow reinstall it and centre it the same?
Thank you

 

[Chris_L@NZ]

Well mystery sort of solved (ish). Other side didn't have this so I'm def tossing it out. As to how it got there in the first place I will never know but I've dreamed up one left field possibility. It looks a lot like a suspension washer so maybe a previous owner had disassembled the suspension at same time and mistakenly matched the washer to the serrations on the piston face and placed it there. It was held on by some gunk cleaned off for the pics. Who knows? very bizarre...

 

[prt57]

I think you are correct in your assumption as I have never seen a washer like that located between the piston and the back of the calliper.
It’s worth having a look around the area to see if you can work out where it should be located.