The 172 SLK 55 has never had the panoramic roof as a standard option, however from MY13.5 the roof panel changed from body coloured to being glass (however you cannot see through this as there is still a fabric roof lining unless the panoramic roof option is chosen).
If you have a glass roof you can change the solid inner liner for one with a cutout. About £200 from a MB dealer and 20mins work. Try slkworld.com for loads of info... Russ