Woodcraft Frame Slider Puck
These solid replacement pucks are sold separately and work on nearly all models of Woodcraft sliders. It's a good idea for racers to carry a spare in the toolbox. They are replaced with the single perpendicular bolt.
The frames on certain bikes are very close to the bodywork, so Woodcraft developed a shorter puck for race purposes. They are 1/4" shorter than the standard replacement pucks.