I have yet to see a 124 chassis with a dust filter but they must exist. On cars like BMW which regularly have filters, I have seen many cars with severe flow problems that were fixed by new filters. In a number of cases the filters were only just past the service interval.
I have replaced 140 chassis filters that HAD to be restricting air flow.
As to the foam, much of the surfaces of the doors are covered with this foam (which deteriates)to keep noise and sweating down. I had a lady with a Volvo pay us big bucks to scour the inside of the evap housing so that the stuff wouldn't blow on her clothes.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician