What Kind of Bolt?

Will anyone tell me what kind of bolt attaches the engine ground strap to the
engine (on a '73 B)?

I tried to removed the nut holding the engine ground strap to the engine-side
but the nut kept turning the bolt. The back-side of bolt is round with not much
shoulder (and there's little clearance between bolt head and engine mount).

It looks like I will have to dis-mount engine and raise it to gain access.
The bolt must have a hex-hole in its head.

as far as my experience, it is a standard bolt. perhaps it got rounded off or someone used the wrong piece.

Posidrive screw, similar to the screw on the door hinge.

Is that not one of the 2 that have a countersunk bolt with a posidrive/phillips head?

On my 74 it's hard to see because of limited space but it looks like it is a crosshead/posidrive bolt. I don't recall it being a problem when I rebuilt the engine several years ago.

Clifton