Mine fell off years ago, and I never replaced them. The cart locks in place with a spring loaded pin in the frame, and those wing nuts were for locking the cart in place at locations other than the default.

What I did, though, being a bit creative, was to drill a couple secondary holes in the frame so it would lock in place at other locations I wanted. Again, no wing nuts required and it only took three minutes with a hand-held drill.

All the best,

Gary