The notch is nice. But it doesn't point to it at exactly midday, isn't that more what you'd expect if it was being used (being chipped out at great effort) for a deliberate marker, not at quarter to? At midday nothing particularly obvious seems to be happening. I would say 'well quarter to is near enough' but the patterns obviously change quite a bit between quarter to and noon?
just an observation.
Local noon is when the sun reaches it's highest point daily, rather than any fixed 'time' of day set at the Greenwich Meridian.
The sun reaches it's zenith at a different time at any given location depending on how far east or west of this 'arbitrary' datum line.