This adjustable stop collar with knurled thumb nut fits our standard guide rods as well as our convex sharpening guide rods.
The stop collar is placed between the stone carrier and the ball and race assembly. It's position is adjusted so that the front edge (knob end) of the stone can't come up onto the side of the blade which would scratch your knife.
This is a handy accessory item that should be in everyone's kit.
Sold by the each. (Rod not included.)