A 4.5mm tall hollow cylinder with a 9.1mm diameter that fits the axle of a C7 rollerskate.
Two are needed for the back wheels of the right skate when removing the back brake.

Unscrew the axle nut and remove the axle, then place the spacer on and re-attach.
Moving the axle to the inner side of the skate will allow the spacer to be less visible.