Short answer: Layson’s sells a filler neck that has been recored to unleaded pump size, so the shut off works correctly.
long answer: Old leaded gas pumps had a much larger diameter nozzle (that fit better in our filler necks). Unleaded nozzles are smaller, and use a clever little mechanical bit to evacuate some of the air in the tank as you fill it into a section of the nozzle. When you’ve filled the tank there’s no more air in it, and gas goes up the nozzle, shutting it off.
well, with such a large hole, you can fill up your tank and not trip that little mechanism until you’re well up into the filler neck, as there’s still air in the neck.
