From 1942-1945 the U.S. Mint changed the composition of Jefferson Nickels. Prior to 1942 the Mint used nickel as the main composition of the Jefferson Nickel. Beginning in 1942 and up to 1945 the Mint began using copper, silver and manganese to produce the Jefferson Nickels, as the government needed nickel to produce ammunition for the ongoing war. War Nickels have a large mint mark on the reverse of the coins so it is very easy to differentiate them from non-silver nickels. War Nickels can no longer be found in circulation since they contain some silver and carry a premium, additionally because of the larger mint marks on the reverse collectors and the public have already set most of them aside.
The 1944-P Nickel is very common in circulated grades since the Mint struck over 119 million of them. In MS60-MS66 condition they are easily available and can be purchased for less than $25. In MS67 condition is where they become scarce with about 4 dozen that survive or so and with no MS68's known.