I would respectfully disagree about the air cleaner set up on that AG. As you mentioned, none of the books show this set up. That looks like a (well done) owner or shop modification done later. But, I would love to be proved wrong.
Do your air vane books make any mention of a breather line? The later parts list gives part number RM-494 for engines with Stromberg carburetors. I'm wondering if the breather line and the aforementioned air horn may be related? As you can see above my earlier AE isn't machined for the breather line.