When the parchmarks of 2013 showed up I crept onto site one morning with a stepladder and ranging rod to take some close vertical photos. The size and shape of the parchmarks strongly suggests that they were for stones rather than just marker posts used to set out the circle, I think.
Recording the parchmarks
The first photo taken
I've been intrigued by the now-famous parchmarks since you first brought them to light 6 years ago. I was then convinced that stones stood in those holes.
Over time I have learned more about the likely practices of the builders during the period the Ring Stones were raised, and was persuaded by a mutual friend that it's possible that these were merely holes dug, then filled in to act as placeholders for stones that had yet to arrive. More, they may never have held stones at all, the people moving on in another social direction.
On the face of it this is a reasonable argument.
But then there's the Avenue, and its existence infers that the culture continued on in a similar, even amplified, vein. Why would they codify the processional approach to a grand structure that was incomplete?
Additionally, even the later Y&Z holes avoid encroaching on the missing stones' locations. If the stones weren't present the diggers of those weird pits would have never known about their positions. And since whoever laid them out did a really crappy job, it follows that they might well have 'ovaled-in' toward the vacant southwestern arc.
If someone put a gun to my head, I would say the Sarsen Circle was completed and the missing stones were robbed out over time.