I have a couple Rex folding lounge chairs:
However, during my move one of the bolts connecting the base to the side came loose and was lost. I took out the one from the other side and I've never seen something like it and I'm not sure where to find a replacement. It has larger diameter shoulder portion above the threads and a narrow. unthreaded region below the threads. It's got a low profile slotted pan head. The narrower lower unthreaded region runs up and down in a groove in the leg when the chair folds/unfolds.
Here is a picture of the bolt:
What is this called? Anyone know where to order one?