Please help me identify it. I assume it's 1941.
The insignia is the Combined Operations badge, worn by various units across all service branches (Royal Navy, British Army, RAF), but primarily by the Commandos. Unfortunately, the "BMB" stamp indicates the manufacturer, rather than a unit, with "BMB" indicating "Briggs Motor Bodies Ltd of Dagenham". The number appears to be a "Heat Number" and represents a batch of steel, and was used to allow a sample of helmets from each batch to be destructively tested for resistance against shrapnel, as the material properties would vary to some extent from "heat" to heat.
The Insignia is of the Royal Marines 3 Commando Brigade, which makes it seem it is at least a representation of a British Brodie Helmet from the Commandos. You might want to take it to a expert to validate it is genuine.