The term "Leathernecks" is a nickname for members of the United States Marine Corps. It was given to the Marines by US Navy sailors during the era of wooden sailing ships. The Marines aboard Navy ships wore high leather collars to protect against cuts to their necks from swords when boarding other ships or repelling others trying to board Navy ships during battle.
Franklin Adreon, who had an uncredited appearance as a Marine officer in this movie, actually served in the United States Marine Corps. During World War II he was a Marine Corps major in charge of a photographic unit.