The American pressed fiber helmet, which was used from the late 1930s until the 1990s, is unique in that it didn’t follow the U.S. Military tradition of naming everything. It had no model number and hence isn’t an M1 Sun Helmet – and for the record that might have been confusing with the M1 Steel Helmet.
During World War II when the pressed fiber helmet was used as a training helmet by the United States Marine Corp it was simply known as the “fiber helmet.” Regulations later referred to it as a “helmet, fiber.”
Further research has shown that the International Hat Company, which continued to supply helmets to the American military at least until the 1970s, actually adopted the U.S. naming convention. Original boxes from the late 1960s indicate that the helmets were officially distributed as “HELMET, SUN, RIGID, FIBER, KHAKI,” yet a period example features an internal stamp noting, “HELMET, SUN RIGID, FIBRE.” Apparently there was still confusion on the official moniker for this particular piece of equipment!
Thanks to Jack at LJMilitaria for providing the photo of the original shipping boxes.