This is a bumper for a v-slot extrusion with an aluminum hinge.
The hinge makes a gap roughly 4mm in between the extrusions, so this bumper is meant to keep that gap equal.
The bumper also has a self-centering notch and is friction fitted.

I included a simple model for a washer. Two of these are used to prevent the aluminum hinge rubbing on itself.
it might get quite tight when installing the washers but it gets rid of the hinge wobble almost completely, makes the operation quieter and no longer grainy.