FWIW:
A. As mentioned, even after speaking to K & N Technicians, K & N recommends but does not specify a certain air filter for Model A's & B's; however, just in case anybody is interested & thinking about combining a Large K & N Air Filter with lots of required air flow, combined with a safer "High Boy" filter arrangement where sold on E-bay, I bought the following items for this arrangement about 5 years ago & it works great with NO "running rich", & NO "plug" fowling:
1. Bought the K & N R-1050 Universal Rubber Air Filter, (ships free from Amazon, today, at about $35.99), which is about the same size as the above Reply No. 5 referenced K & N E-9257 , (i.e., 4.6" O.D. x 7.1" tall), and has a "closed" top, and a protruding open flange on the bottom which will correctly fit the "Hi-Boy" filter arrangement sold on E-bay.
2. I also bought the carburetor connections for both the Model A Zenith & Model B Zenith & found that the K & N R-1050 air filter combined with the "Hi-Boy" fits well with filter upwards above the carburetor & under the glass sediment bowl on my 1930 Town Sedan with either a Model A or Model B carburetor.
3. The clearance between the closed top of this K & Nair filter to the bottom of the metal clasp bolt on the glass sediment bulb is about 15/16", which is ample for either glass bowl removal, or air filter removal.
4. The filter is positioned directly under the sediment bowl and very close to the Fire Wall & aesthetically blends in as opposed to hanging out & appearing like a dangling participle on a University English theme.
5. As far as Fire Safety, (just in case if anyone "wants" to consider Fire Safety), with this arrangement, the filter is positioned up high & can never get saturated with liquid gas possibly dripping from the carburetor; and it is also is positioned up high & far away from the hot manifold surface and hot exhaust surface.
6. Someone reported in the past that the "Hi-Boy" arrangement gets loose & falls off; however, if one has the mechanical aptitude to tighten rubber hose clamps with a screw driver, it will not fall off or even became loose if tightened properly.
B. The only drawback I can think of with this arrangement may be that with clean plugs, better gas mileage over paper filters, and far less piston ring wear ....... that the plug, gas & ring manufacturers will never send anyone a Thanksgiving Card every November.
Hope this helps someone one day searching for such a K & N/ Hi-Boy" air filter arrangement.