Black rubber washer 13mm inside 23mm outside 
This product is ideal if your wanting to install a lampholder to a pottery or glass item.
Generally the best way to do this is to use an all thread road and nuts either side, using this rubber washer will ensure you don't put a lot of stress on the pottery/glass item.
As the rubber will absorb some of the force from the nuts.
Inside diameter: 13mm
Outside Diameter: 23mm