Look at the curve the top of the drivers fender makes over the tire, then look at the front passenger fender and you will see instead of a curve over the tire it makes more of a flat line across the tire. Once you see it, it becomes very obvious.
Well with all due respect, I asked the seller. I was told it is a fender from a 1955 300. That mismatched 300 fender was on the car when the current owner bought it and the person he bought it from had it for over 25 years and the 300 fender was on it when he bought it, and he didn't know why...