Hummm... My guess it's pressed on with a keyway. If your can't see in the hole with threads it's pressed on. No Bolt head, Allen wrench head etc. have to be. Put a gear puller on it. Have to come off with it. Can't see down in the hole so that would the tell tale. JMO..
While I am looking for the filter housing, does anyone know how to separate these front PTO parts? Is the Clutch splined, keyd,or threaded on? This one is giving me a hard time.
I have a new Yanmar original air filter housing on the shelf. I can PM you with pictures and price.
I also have new Yanmar front PTO pulleys, bearings, and clutch.