[Pwhoever]

That #20 filter is probably the little D-shaped filter that is inside a rubber cap that fits on the lower part of the air compressor. If you take a look at the first photo in THIS THREAD, you will see the rubber cap on the lower yellowish colored part of the air compressor. That thread is also a great guide on how to service your air compressor.

 

[Pwhoever]

And you missed one more, it is 52718-MN5-003 #12 on the diagrams and goes in before the dessicant gets put in the compressor.