I have one of those in my 56 and it works pretty good too Got mine at Ecklers... Later, DaveI put one of the check valves in the hose connections between the 2 fill pipes going to the tank. It has prevented gas splashing back out when you fill the tank.
Several of the suppliers offer them.
That part is true, but on a 57 it is supposed to have a vented cap that does prevent pressure. If not the gas tank will buckle. I learned that one by first hand experience.The air vent in the 57 filler tube is a "closed loop" kind of thing - it allows the fuel to flow down the tube without "burping", much like the handle on a milk jug. I wouldn't expect it to prevent pressure buildup.