This was on my 57 so it fits many years (I think till 72?). At any rate, here's the story. The gentleman I bought my truck from had this on it. He said that he thought he remembered that Chevy did a few special edition right side spare fenders. He struck up a conversation with a gentleman at a car show about this many years ago. This gentleman said, not only did he remember the same thing, but knew where one was. It was purchased and used on his restoration in the early 80s. It remained on the truck until I had it restored last year and went to a fender with no spare. It has a little rust, but nothing that can't easily be fixed. This appears to be a factory job, but I won't swear to it since I can find nothing to support that Chevy actually did this other than a few stories. At any rate, it makes for conversation. $100 email me for pictures.
A couple of questions: Where are you located? Is it a long bed or a short bed fender? I tried to send you a PM but it appears you have that option turned off? I would like to see some photos of the fender, especially from the back. Thanks... John