There are a number of different variations in the US Lincoln head penny over the years. Before World War II the penny was made consistently with copper and other alloy metal, including bronze alloy.
But during World War II the need for copper was high for the use in ammunition, and other areas for the war effort. The U.S. Mint tried different metals to try and substitute for copper in the penny.
They tried plastic, they tried steel variations, including galvanized metals, but each thing that they tried didn’t work out. They finally worked to use steel for a while during the war, and after World War II they eventually returned to the composition including copper that was used before the war.
But during the War, in 1943 there were a few pennies that made it into circulation that were actually copper, during the time when the pennies were made from steel. An estimated 40 pennies were believed to have been produced, with 12 that are known positively to be in existence.
They are extremely rare and they are believed to have been made by accident, such as copper left in the machine when it was changing over from copper to steel. They are so rare that in 1958 a single known Copper 1943 S (for San Francisco, the mint where these rare pennies were produced) sold for an amazing $40,000 dollars.
That was in 1958, over fifty years ago. In 2004 another one of these pennies sold for $200,000 dollars. These are coins that people have tried to counterfeit over the years, but there are several methods that can be used to tell if they are not real.
The 1943 Copper S penny is one of the most rare coins in circulation in the United States. There are 12 that are known, with an additional 28 that may exist, but no one knows for sure, the number of 40 is an estimation based on the theory of how they were accidently produced.