In 1943, pennies were made of zinc-coated steel because copper was needed during World War II. At the beginning of 1943, a limited number of copper pennies were struck by mistake. In honor of the Mint’s 225th anniversary in 2017, pennies made in Philadelphia had a “P” mint mark for the first time.
https://www.usmint.gov/learn/coins-and-medals/circulating-coins/penny

