Activated charcoal pollen particle filter. Could be used in 986 / 996 models without M571 option too.
Porsche 986 Boxster / Boxster S ’97-04
Porsche 996 C2 / C4 / Turbo ’98-05
Porsche 987 Boxster / Cayman Mk1 / Mk2 ’05-12
Porsche 997 Mk1 / Mk2 ’05-12