>> Considering for a moment the possibibility of a non-ritual use - could they be direction indicators,
>> maps (of communities or barrow cemeteries) or even star charts?

I can match the CnRs on one stone particular stone I know to a map of the mountains around it. More rings would indicate higher peaks. The distribution of the CnRs on the surface match quite well to the layout of the hills.

Sadly I have never found a similar correlation anywhere else, so for now at least I have to believe it's a coincidence :-(