From the Laser Scan Report:
On New Year’s Day 1954, Dr O.G.S. Crawford identified a carving of a small dagger on the SW side of Stone 23 (F364, see Figure 14). This carving is commonly illustrated as appearing on the SE interior face of the stone as Crawford simply described the carving as being on the S face (e.g Cleal et aI. 1995, 30, Fig.17); this is incorrect. A superb photograph of the carving appears in Crawford’s 1954 article on the carvings that he discovered and this shows a short blade with a clear handle and pommel. This carving exhibits a deeply incised outline, but the central area has not been lowered by pecking
This technique of manufacture was not employed on other prehistoric carvings, but there is no reason to suspect that this carving is not prehistoric.
Comparing the 2012 close up and the 1954 one it is noticeable how recognisable the Lichen patches are, and how little they have changed. Emphasising how slow growing and long living they are.
UPDATE - New photo taken by torchlight to show details: http://www.sarsen.org/2012/10/new-photo-of-stone-23-dagger.html
Click pictures to enlarge.