>capstone
Q might have called it keystone though
Might be the Western side?
"Astronomical features
The four outer stones are oriented to mark the limits of the 18.6 year lunar declination cycle.[20] The center column features a hole drilled at an angle from one side to the other, through which can be seen the North Star. The same pillar has a slot carved through it which is aligned with the Sun's solstices and equinoxes. A 7⁄8-in (22 mm) aperture in the capstone allows a ray of sun to pass through at noon each day, shining a beam on the center stone indicating the day of the year.[2]"
https://en.wikipedia.org/wiki/Georgia_Guidestones#Physical_data
There are two videos from two angles from what I saw
Location of that light appears to be approx where the 2nd camera could have been, if I am not mistaken