Hi Mike.
I do not have written proof. This is my logic based on what I have been told/observed. Am open to disagreement, being corrected.
When I was in high school I had a mid sixties Honda motorcycle where the petcock developed a leak. It had 3 positions - off, reserve, and on. So I drained the tank with the petcock in the reserve position expecting it to drain dry.. Then removed it and surprise, got about a cup or two more gas. Examining the petcock it had a very short tube sticking up into the tank for on, and a longer tube for reserve. I asked the motorcycle dealer/parts guy why the short tube for on, and he told me it was so that water/rust would not go into the petcock screen and plug the screen when bike when the gas was low. This bike had a screen similar to the gas bowl screen in tan A, but was inside/part of the petcock.
I know you have old tractors, my dad also collected old tractors. So I asked him about it. He told me some tractors have a depression at the bottom of the tank to collect water/crud, and they are bear to clean out. Additionally he told me some tractors the hole for the petcocks had a flange sticking up creating a place in the bottom of the tank that did not drain with the petcock removed for the same reason.
I have also noticed when draining my A tank and radiator, any rust/junk/water immediately quickly sinks to the bottom of the tub being drained into, and when agitated by moving/shaking the tub has a tendency to stay on the bottom instead of floating around suspended in the gas/coolant. Since it tends to stay at the bottom and if the filter has a bottom part of the screen that is solid, I assume there is less chance of plugging the screen.
Your point of only the bottom of the screen getting plugged with more screen available towards the top, to me would work better if the fashioned screen went higher than the commercial screens.