1945 P War Time Nickel Transitional Error.

How can you tell its not silver? A sure test. Place another silver coin and a current nickel together and cover with a single ply facial tissue and check the colors to if they are same or not.

Two words. "Dumb planchet".

A dumb planchet results from internal flaws such as bubbles or laminations (Whichthe War Nickels are famous for) which prevents the coin from "ringing". You note that it just goes "thud", even regular copper nickel alloy five cent pieces don't just go "thud". Color or appearance are also not sufficient tests for determining the composition of your coin. To be sure it is a non-wartime alloy you are going to have to do specific gravity tests and preferably an XRF test on the coin. So until proven otherwise it is just a regular warnickel with internal flaws.

Oh and by the way, even if it is a copper nickel coin and rarer than the 43 copper cent, that has no relationship to its value. Even if it is copper nickel I would be surprised if its value got out of four figures, even if it is the only one known. I have a couple of unique pieces that I purchased as unique pieces that I paid less than $300 for. Extreme rarity doesn't always mean extreme prices.