'Thom described them as being more like a spiral than a circle'

7) Z&Y holes, a circle, spiral, or an arc ?

Attempts to impose a circle as in the Wikipedia diagram -
( http://en.wikipedia.org/wiki/Y_and_Z_Holes )
are not able to follow the holes as accurately as the arcs are. The Y and Z holes are not a circle or a spiral, they are two arcs. (as defined by maths geometric shapes) The Eastern arc is just too regular to be an accidentally irregularity.