As he reported on his blog recently Mike Pitts found two modern screws, just a bit corroded, in two of the sarsens....one is in the small upright no. 11.
Here are some close up pictures of it, it is a mystery which no one seems to know about. Click any to enlarge.
I previously published photos of what I think is the other "screw" he mentions, which is in Stone 52, here.