The flapper doors in all the Mercedes are covered with foam. This foam deteriorates over time and pieces will come loose and blow out the vents. The door is metal or plastic and the foam problem will not interfere with the running of the system.
Below is the fresh air flap door from the 82 300TD AC system.
This is a metal door that is covered with the foam. The foam quiets the airflow noise and makes the shuting of the door against the heater box case, quiter.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car