It is a white L or S on a green background:
http://www.hankstruckpictures.com/pix/trucks/len_rogers/july2003/mercedes_titan.jpg
This one has both:
I found this reference on a trucker’s forum. The symbols seem to relate to Austrian regulations about low emission diesel engines. Hope this satisfies your curiousity. 
[url http://www.trucknetuk.com/bbsuk/Forum1/HTML/000291.html /url]
The “L” on the front shows that the truck gives low emmissions of exhaust fumes and the “S” means that the truck has got as near silent engine as poss.They only apply to Austria , when you travel through Austria as well as rd tax you have to have an "ECU " points form. The lower the emmissions and noise the less points you need, also trucks are banned on Austrian rds after 22:00 hrs unless your vehicle has the “L” AND “S” stickers. You can not simply put them on because you have to get a certificate from the main dealer to confirm that the vehicle is as described, you must also carry this certificate with you for inspection.