Damper retaining bolt?

Thank you. I guess the guy that rebuilt it in the 80’s probably left his installer on.
Or enjoyed using it to roll the engine around to set/check valve lash, keep it, and put a thin smear of grease in the crank threads so they don't rust. You may want to pop it back in to do just that, loosen fan belt if blades don't clear. They are a TIGHT press fit, You're good.