This is a jig you bolt onto a piece of wood for clamping.  It lets you place a dowel , I'd say up to 1/2" and rotate it in the V groove against a sander at an angle.  The stop prevents it from moving forward so you can get a consistent depth of cut or chamfer size.
I added two holes to let you screw it to a piece of plywood or mfd so you can clamp it to any type of sander you need.