If the coin weighs less than a standard nickel, someone with too much time on their hands smoothed off the back. If so, it would be considered a damaged coin worth only face value. If it's the same weight (5.00 gm) as a normal nickel, there is a slight possibility it's a minting error that resulted when two blanks got stuck in the coin press at the same time. One blank would then be struck with the front image and the other with the back image. You'd have to take it to a dealer who specializes in error coins and have it examined first-hand.