What is this cylinder covered in threaded studs and what is it for?
It's the inner liner of a heat muff, either for cabin or carb heat (you can see the engine mount tubes right next to it and that's exactly where I'd expect to see it, and the black pucks behind are part of a nose gear shock absorber).  It could be home-made, using threaded rod welded to the inner liner, or maybe it's a manufactured component with the rods swaged in place like rivets.
An outer shroud will go around it with inlet and outlet bosses for SCAT hose running to the air inlet and outlet to the cabin or carb.  Sometimes springs are wound around, or some other metal that can act like cooling fins; some builders just stuff the space with stainless steel wool.
The threaded rods are to improve heat transfer from the inner shroud to the air flowing past them (the threads maximize surface area).  There will be a flange out of view at each end that creates an enclosed annular chamber.  The inner shroud itself typically has internal baffling to function as a muffler for the exhaust.  Crude sketch of what I mean below.  My own plane has this kind of thing, but without the studs, just an open space.