It seems as though the capstone edge is in exactly the right position and has just the right curvature to allow for different elevations of the Sun at different times of year.
With respect, that seems most likely to be serendipidy rather than selection or carving and as such can't really be cited as "evidence" in support of what you're claiming.
In reply to Rhiannon yes, the midday sun varies in elevation through the year so if you want to know midday each day it would probably be simplest to just scratch a line, like in sundials. Whereas to me, Midday marked by a point (or notch or lightbox arrangement) implies an intention to mark midday on a particular day rather than every day.
But I'd go further in my "what's most practical is most likely" musings:
I'm thinking a shaft of light shining through a gap wouldn't be my timepiece of choice as if the gap was, say, 5 suns wide, the light shaft doesn't move very well and ought to be much narrower and it would be a real headache to fashion the gap so it worked every day. So shadows are best for marking the time year round - at midday the sun travels about one solar diameter per 2.5 minutes and the shadow reflects that rather sensitively all year round.
The (reproduction)Steyning sundial...just outside the library, should anyone visit..is a good example of this.