I heard a weird rattling noise coming from the right rear tire and found this washer floating in the space between the rear rotor and dust shield. I don’t recognize it and couldn’t find it on a rear brake exploded part diagram. Anyone recognize what this is? The part measures 31mm outer diameter and the inner is 13mm and it’s 1 mm thick. Thanks in advance.

 

It is the washer that goes between the caliper mounting bracket and knuckle. Honda 90651-SWA-000

See at the video starting at the 7:30 mark:

 

thanks for your help and part number! Really appreciate it.