2008 Boxxer World Cup rebound knob

the 'bolt' is normal (I think, your description is not so great). The fork comes with a knob that snaps on to the lower fork bolt. THat knob has a 2.5mm wrench extension that fits into the 'bolt' and turns the adjuster mechanism.

If it truely does not adjust the rebound speed (while riding, not bouncing in you kitchen or parking lot) then there is something wrong. Have you checked the damper oil level and viscosity? have you checked the proper sag, top -out, air pressure, and bottom out? Have you checked the lower bushing lube? have you checked the seals and the required (slick honey) seal grease. THis stuff should all be done every 20 hours of riding......

#1. You have a Boxxer World Cup.

#2. Are you using a 5mm allen key or a 2.5mm allen key? Like Davep said you need to use a 2.5mm to adjust the rebound, the 5mm bolt on the damper side of the Boxxer is hollow to access the 2.5mm rebound ajuster.

That's great, thanks for your help guys. I was using a 5mm allen key and thought this was all that needed to be turned. Obviously nothing was happening, but I have taken that all the way out and used the 2.5mm allen key which has adjusted the rebound.