|Click to enlarge
Stone 23 has several interesting surface treatments. At about shoulder height on the outside facing the path there is a highly polished area, quite different to any other stone surface in the monument. The enhanced picture shows it to the right. It looks like a polissoir area.
I think it predates the 19th century graffiti of Mr Senior but others aren't so sure.
For more pictures of Stone 23 see: http://www.stonesofstonehenge.org.uk/2014/05/stone-23.html
As an aside I think the upper graffito is W.READ rather than WREN.