Hi Gregg
The parts manual was correct, you can use the 1958 part in either the 57 or 58 car. I'm sure the reason for this is Ford could stock one part that would work for either year. If you have the 57 inter fender you would have to cut the notch for it to fit on a 58 car. That's why the manual shows the 58 part with notch will fit both model years. In both cases there is a rubber weather strip that seals the inner fender against the bottom of the fender.