Welcome to the forum!
I don't know the size of the bolt; but here's how I do it...
since it's kind of a small bolt: I go to any hardware store, and pick up a couple of likely candidates.
(It's a pretty standard bolt size and threead...)
Try them on for size, and go get more of the one that makes the magic happen!
Good luck, and Happy hunting!
(When I took the one jackknife sofa out of mine: I promptly lost the bolts... It was easy to find replacements. )