The story
From the helmet's appearance similar to a coal scuttle (coal bucket) of the early 20th century.
Etymology from Wiktionary, CC BY-SA 4.0.
coal scuttle helmet
Today it means: A style of soldier's helmet that is almost identical to resembles the helmet used by current Chilean soldiery, German soldiery at the end of World War I and during World War II, and Chinese soldiery during the Second Sino-Japanese War. Full definition →