I toke this in on trade knowing that a bolt was missing, never guessed it would be a bolt I can not find. 3/4-16 left hand thread. would like to know who made it, to see if I could get a bolt from them.
Try McMaster-Carr or Grainger for your bolt. I am lucky to have a local hardware store that can order any odd ball bolt I will ever need, so if you have a store like that (not any of the national "home Centers") you can always ask. Oddest one I've needed to date was a 9/16" fine thread bolt 4 1/2" long and I got it thru my local guy.