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the bigger gasket of the two is for the turbo to downpipe join. You need this.
the nuts are for the turbo to downpipe join, you need four.
the studs are for the turbo to downpipe join. You may need these, depends if any snap while taking the old nuts off
the decat you have has a join half way along for ease of fitting. The gasket that goes in between these joins is missing as per the auction details. You won't find these at the dealers as the standard pipe is one piece. Use paste in there instead of the gasket.
You will also need to find 3 nuts/bolts yourself for this join from a hardware store. Don't use nylocs in case these get too hot and melt. I would get a bolt and two standard nuts so you can lock them on