Sorry John, only a guess here, but it looks to me like maybe an 8mm Buttonhead bolt you're missing there.

(BTW, word is the people who saw you on the highway today said that that was the absolute WORST Kevin Bacon impression they ever saw!...get IT???..."Footloose"!!!....oh, never mind!!!)

Yep! Just like a good Single Malt Scotch, you might call me "an acquired taste" TOO.(among the many OTHER things you may care to call me, of course)