Calver stone-with-a-hole

On our jaunts around Calver I keep seeing this beautifully re-furbished house on Main Street with a strange stone-with-a-hole built into the gable facing the road. Is it a megalithic sun-worshiper's sextant or perhaps a Roman carsie (they did a lot for us in the Derbyshire Lead Mines)... I wonder. Anyway, thought I'd draw it.