Just done some maintenance on my rear callipers and when moving the car back into the garage I have found this on the driveway where the S was.

Does anybody recognise what it is?
Is it a bung/seal for the brake reservoir cap? It does fit the underside of the cap but want to make sure.

Cheers

 

Sure it fits in the cap and seals against the reservoir.

 

Same thinking.. sits under the resovoir cap. Check see if yours is missing.

 

It does fit the reservoir cap, my dad mustn’t have noticed it dropping off when he removed the cap.
Thanks.