This is the only w212 pollen filter thread I can find on here. Just done the filter on my w212 so thought I would chip in a thought.
Obviously the metal retaining clips were missing, and because the Bosch filters have a foam sealing edge, this makes it virtually impossible to slide the white retaining clip across with one hand because the filter is trying to push itself out of the housing by about 5mm.
No problem though. I used some circa 5mm width cable ties, one on each side where the metal clips should be, to pull the filter fully into the housing. Makes sliding the white retaining clip in place much easier.
Next time I need to replace the filter, I'll just cut the cable ties and replace.
The filter was pretty filthy, noticeable by the weight of the old dirty one.