filler neck modification

I've been watching this thread for a while, but IMHO I don't believe the true cause of this issue has been identified. I don't see how a flapper valve has any ability to prevent this issue.

It's been a few years since I've owned anything but an early A, but don't the later As have a gas tank vent tube that connects towards the top of the filler neck? Any chance the lack of this on the early As is causing the issue?

I have my early A filler neck out of the vehicle, so I can post some pictures for the end that goes inside the gas tank. Maybe a later A owner can post a photo of theirs in that area so we can see the difference.

I plan to test this theory by tapping my filler neck and running a vent tube down to my fuel cell, but I'm a long way from being ready for accomplishing this.