Sold as a pair (2) of slop stoppers.

100a machined urethane

These are made specifically for the Caliber II reverse kingpin longboard truck.  

These fit can fit over the top of your board side bushing before you put the hanger on.  We have even seen them on the inside sides of the hanger in between the road side busing and the hangar with the plug facing down.  The most important thing however is that the raised segment goes between the kingpin and the inside edge of the bushing seat.

That raised plug edge is about 4.25mm wide.