i'm at work and will get home around 5 pm -EST.
Here is a link to my photostream if you want to see the photos I have of the XS
Opened up Photoshop and blew up the left stem image I have. From what I can tell, left stem on these pair have the O- symbol and -20- after it. I have not ever owned a pair of XS, so I'm not sure what to make of it.
http://www.flickr.com/photos/78736197@N02/