This is the bolt that links the top of the uprights to the keel. It is often bent if you have a heavy landing. The way to check whether the bolt is bent is to remove the bolt and roll the shaft along a flat surface. Looking from the side, along the level of the table, if you see the bolt moving up and down as it rolls then it is bent.
NOTE: We always recommend replacing the Nyloc nuts with new if they have been removed, as re-used Nylocs will not have such a secure grip on the bolt thread. We also use a locking compound, such as Loctite, for additional security.
Why is it so expensive?
Unfortunately we can't get these bolts from stock, they have to be cut down to the right length and then re-threaded to accept the correct size nut, which makes them more expensive than a stock bolt.