A lot is being made of the "hole"...one poster, in the other thread alluded to it being for the erection of a maypole(yes I know it was a tongue in cheek remark)...but it might not be that far off the mark....perhaps(and only perhaps) the hole was used as an aid to building....by inserting a sturdy lever-style piece of wood(this could explain what was described as a "squircle" too as this would give a good grip of a piece of timber) for aiding in raising the slab and moving it into and holding it in position until the wall structure was fully stabilised...has anyone considered a simplistic theory??
One of the theories put forward was that it was used to haul it up with ropes Resonox, but we just don't know do we. To me, as a pure guess, it was 'drilled'out afterwards as it is perpendicular to the slope of the Capstone. Of equal merit would be to say that ropes were tied to it then secured in the ground like a guy rope to prevent it slipping off. Let's face it, everybody who sees it can't believe that it actually stays there! We may never know now.
Of interest is that it is 'drilled' at rather a thin place. Now whether it was always thin there or the lower 'layer' of granite fell away as it was punched through is open to further question but you can see quite clearly where it has fallen away from.