The grommet indicated by the blue arrow in both attached images fits into the bracket indicated by the blue rectangle to support the rear brake hose.
Martin is correct. It goes onto the lower rear caliper bolt to hold the speedo sensor grommet.
The 4 swing arm to final drive bolts are all just an M10 bolt and a washer. No brackets.
It looks like the bracket that fits on the caliper bolt that secures the grommet for the speedo sensor wire. Maybe you found a spare when reassembling.Regards Martin.
So the previous owner(s) must have run out of flat washers but had one of these brackets lying around instead.