Ads aren't always correct Pete regarding the air filter but as far as I can tell the big black filter was fist released on the 3rd model Special ,the base that has no alloy extensions for the front or rear flap to pivot off.
The Patent for that filter came out in September 1958 so this filter wouldn't have been used on earlier models than Sep 1958.
I would just build your mowers how you like Pete as most early mowers are never going to be 100 percent original, now you can only make them look close to original.
