Agree a PO "modification" was done. But having said that, it's interesting that I think my '71 driver's wheel well (on the top) had a couple of odd shaped holes that defied definition too.
Too odd to be a drill hole, and not punched. I always thought it was where a stud was attached for the jack (similar to the one you have in the pic for the jack handle?) and did not think much more of it.
Never even thought of burn-through! But since it's not near a joint or component mounting point, I didn't think much of it, other than the missing stud theory.
I'll have to go compare to some others now.
Makes me hanker for the next get-together when 8 or 10 of us can hang out under a hood, compare notes and make rude comments about other Broncos!
Paul