The British Army adopted this form of helmet, initially known as the Brodie’ helmet, as its standard head protection in 1915. However, it was not until spring 1916 that it began to be issued to troops in large numbers. Many were put into storage and reissued at the outbreak of the Second World War. Modiﬁed but similar helmets were worn by British soldiers even until the 1980s.
The wrinkling on the top part of the chin strap in this sketch appears to be webbing, rather than leather. The sprung webbing strap was not approved until 1936, meaning this portrait likely dates from the Second World War.
With thanks to Emma Mawdsley of the National Army Museum