An original British Brodie helmet. This is the Mk.2.,referred in it's entirety as Helmet, Steel, Mk.II, No.2.
The Brodie helmet was standardised during the First World War, but continued in use until well into the later stages of the Second World War.
This helmet is the Mk.2, introduced around 1938 and bearing more or less identical to the Mk.I, but differing in the chin strap and method of chin strap attachment. Later Mk.2s had a different liner.
No maker mark is visible but there does seem to have been elements of green paint added over the black, subsequently removed. Remnants of liner and chin strap.