That looks like the Sky Dynamics 4-into-1 exhaust that my previous RV-8 has. Sky Dynamics can supply you with the bolt, spring, washer, and nut as an assembly. Buy some extras, as I had to replace a few of those when I owned the airplane. Inspect and/or replace those regularly, as the bolt would be worn by the bracket and fatigue and break off just above the nut, with the loss of the nut and spring. It only seemed to happen to the ones at the 4-into-1 exhaust collector.
skydynamics.com
My previous RV-8 with the Sky Dynamics 4-into-1 exhaust:
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