This is the extended center J-bolt only for the SuperTrapp 5S Series Standard 4 Inch Resonator (Part No. ST-5S-5500). The resonator includes this long bolt, but replacements are available if yours gets damaged or goes missing. Sold individually; includes washer and nut.
This bolt is normally only used with the resonator, but installing more than 12 diffuser discs without the resonator will also require an extended bolt. This bolt is not threaded all the way down, so you will need to add washers or a spacer under the nut to keep the discs tight.