Early in World War II the U.S. forces were still equipped with the M1917/M1917A1 "Doughboy" helmets that were left over from world war one. In 1941, the M-1 "steel pot" helmet was adopted as a replacement in the armed services, although it did not become universal for at least another year.
The M-1 Steel Helmet became the symbol of U.S. military forces and was used world wide by all branches of the services for the duration of World War II, Korea, and through the Vietnam War.
Several distinct characteristics can be noted to help determine the age of an M-1 helmet. The World War II examples will have a seam in the front whereas those produced post-war will have the seam in back. The seam that is being referenced is located around the rim of the helmet. Also, early production helmets are found with fixed bales. It was not until 1943 that a swivel bale was introduced.