This is the last pic I will show as to 'reveal all' just defeats the object of writing about it with a view to publishing. Here is a clearer view (and I have better ones) showing the knib again. We are on the same wavelength here SG as that is my belief also. If a shallow socket as described was made and the knib used as an aid to line up the capstone, then that would explain why the capstone at that point has broken away underneath as clearly seen from underneath the 'hole'. This would have happened as the sheer weight of the cap began to slip. No mystery attached, pure old-fashioned common sense!