It's called a shoulder bolt. Plenty sizes available. Google shows many for seat belts. May or may not work. Determine thread size, shoulder diameter and shoulder length. Then try McMaster-Carr or equivalent. I can't remember if it had a nut or threads into seat? A bolt and sleeve may be possible. I never say "make one" because most don't have a lathe and such.
See 3.00 Guessing shoulder is .500 Diameter, .500 long and threads are .375-16.
Example: McMaster-Carr 91259A707 Not hex head. Call it custom
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