Those like me who can’t find or can’t get to the Polisher Stone at Avebury can take about 1% comfort from a mini version I found at the West Kennet Avenue (low down on one of the 2 stones closest to the road but still on the Waden side). Three definite sharpening grooves. But only about 4 inches long.
http://www.themodernantiquarian.com/image.php?image_id=19070
Age unknown (though one has been scraped at recently)

An ill favored thing, Sir, but mine.