Error Or Fading?

A weight will tell you almost everything you need to know, a normal nickel weighs 5 grams. It is definitely not struck on a cent planchet because it is too large and has full design details. As far as I know, foreign production was only at the Philadelphia Mint(San Fran was also minting during WWII). Very very slim chance it is an off metal error, weight will determine that.

tershaffer - Just to be clear about something I saw in your post.

"Casting" is specifically a term used to describe the process of pouring something liquid into a mold to harden. This is not how coins are made. Coins are actually made with cold blanks of metal that are pressed with tons of force between hardened steel dies. This process is called "minting", and coins are "minted".

The predominant reason why casting is not used to make coins is that it would be far too easy to counterfeit coins that were cast with minimal effort and expense. In fact, coins are counterfeited by casting, and the difference between that and minted coins is pretty obvious. A press capable of minting coins would require tens of thousands of dollars worth of equipment and a pretty sophisticated machine shop, annealing ovens, etc.