By Ian Simpson WASHINGTON, March 7 (Reuters) - The Cyrus Cylinder of ancient Persia, a landmark in religious freedom and a potent symbol of Iranian national identity, will begin its first U.S. tour with an exhibit that opens Saturday in Washington. The barrel-shaped clay artifact, 2,500 years old and only 9 inches (23 cm) long, has been described as the first declaration of human rights and an influence on leaders from Alexander the Great to Thomas Jefferson.