Bob, I just looked at it again, and you are correct, the screw in question, which is the one that holds the hammer on, does indeed have a slight taper or "v" shape to the slot.
I guess that is why my Grace screwdriver, which has parallel sides, does not bottom out in the slot.
Sorry for the confusion. It is a thin slot, perhaps .024 or around there.
My apologies for the inaccuracy of my post. I had to look really close to see the taper.