Until recently, I've bought my filters from MB dealers. They always seemed to be made by Mann.
I recently bought filters from a supplier used by a lot of MB Club members. I bought one Knecht and one Bosch. The Bosch was identical to the Mann filters I had been using and it was BIGGER than the Knecht. The Knecht has 6 smaller holes and the Mann/Bosch have 4 larger holes.
Back to my original question, I wonder if the check valve is actually in the filter mount rather than the filter.
That's why I posted this question.