The 2009 model machine needed a new carb., but the original type (ZAMA C1M-S141C) wasn’t available. A replacement carb. (ZAMA C1M-S261B, on left in photo) had an extra pipe to direct vented fumes into the air filter. This required a modification to the air filter housing, which was already marked-up with hole position. See video on Vimeo for more details. This is likely to be the case on other Stihl machines, as a result of  tighter emission control laws.

This is the modified air filter housing which accommodates the extra pipe (top left).