The stone (arrowed in the picture below) is very clean in a band in the middle, this is usually thought to be from the cleaning off of graffiti but I think there may be another explanation.
Stone 23 unexpectedly fell over in March 1963, and 1963 was when the centre of the circle was gravelled. It was put back up in 1964.
In the pictures below, from Hawkins' book Stonehenge Decoded, it can be seen how the clean area was lying on the ground for that year and the bottom postcard, copyright 1970, shows the distinct band of clean stone. I think the fallen stone may have rocked a little as people stood on it and that area was sand blasted clean by this action of the stone on the ground.
As usual click to embiggen