Hi Nick , I think I know one of the stones you mean . It has been there at least 18 months . Just past Lurglomond on the north side of the road , I have some pics and am pretty sure it was a recent bit of sculpture by the fermer.
I have an abiding impression that neither of them were that far up the loch, Lurglomand is just a mile or two west of Acharn and I'm sure both of these were well before that... As I said, not having my usual gear with me didn't help, I'll have to crack soon and get one of those damn GPS things...
Both stones looked "right", if you know what I mean. Not sculptured in any way, just old and weathered.