Regarding my post above, I can now confirm that the Evernew bladders fit the Sawyer filter perfectly. So I will use Evernew as my 'dirty' bladder from now on.
As you can see in the picture below, the threads are very different (L:Platypus, R:Evernew). Platypus is the odd one out. Evernew and Sawyer use the same industry standard thread, similar to most softdrink bottles.
Platypus + Sawyer might
work for a while (in fact, I used that combination on my AZT thru-hike) but it won't work reliable (or at all) especially over time when the rubber seal inside the Sawyer gets compressed and leaks start occurring. As the picture shows the thread on Platypus is wider so when you try to screw the Sawyer on it gets stuck before a reliable seal is established.