Aside from some very specialized parts, 99% of all OE parts come from an outside supplier.
It's just that NOT all of the parts manufactured meet MB's specs. However, some of the MB's specs are NOT, IMHO, critical, like filters/consumeables (e.g., brake pads, etc.)
While I've always used OE tranny filters on the 500E, after looking at the OEM (not aftermarket) filter, it had all the right valves, dimensions, etc. I'm going to use it because it's significantly cheaper.