The pie tin on the back wheel that keeps dorks like me from shifting the chain into the spokes. Usually, adjustment is all that is really needed, but...this one was kinda cool, because it is an integral part of the earliest freehub that I've ever seen. I'd need to find a spacer to keep the cogs tight without it. And El Twe woulda had to find something else to dish out crap about...always thinking about my fellow man, that's me...