In Reply to: Air cleaner for '39 posted by Kirk-NJ on July 25, 2017 at 19:41:47:
The early 9Ns used both United Services and the Donaldson air cleaners; there was no cutoff/changeover date whatsoever. Here's the March, 1940 first 9N MPC LINK from our MANUALS forum. Scroll to the Crab/Air Cleaner page. Note the *STAR* symbols next to each part number. Now flip to the back page and it lists all the deviations by serial number and what the specific symbols signify. I'd say your 9N used the United as you have but might have used the Donaldson - I didn't look it up by your s/n.