These are black plastic push in plugs that go into the 10mm threaded holes where motorcycle rear view mirrors would thread into.
NOTE: You might have to get a pair of scissors and shorten these to fit - check depth of hole before you stick them in that hole.
Once you stick them in, they are very difficult to remove without destroying!
For example, some BREMBO master cylinders have this threaded hole in the clamp, others have it in the master cylinder body, and some clutch perch assemblies have a threaded hole for a rear view mirror in them as well. These black plastic plugs just push into those holes to clean up things. They look nice.