It's just a small, usually plastic, piece that looks like the end of the tranny shaft. You push it through the clutch plate, after installation, until you get it to seat in the ?pilot bearing? and that ensures your clutch plate is centered on the flywheel. On other vehicles that I have replaced a clutch on, it has always come in the kit with the clutch disc and pressure plate. If not, they can be bought seperate from a parts store for no more than a couple of dollars for the plastic one. They also sell metal ones for you to keep and use later, the plastic ones are intended to be throwaways. You can also use the transmission shaft cut off an old transmission if you have one lying around. Check out this link.
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the picture, its the cylindrical thing with the ring on the end.