This is a British Mark I helmet which has been issued & used by a soldier in the 2nd Battalion, 9th Infantry Regiment, 2nd Division.
There is a 2nd Division insignia painted on the front, with an indian head star over a yellow pentagon, signifying the 2nd Battalion (Companies E, F, G, and H) of the 9th Infantry Regiment. The insignia has some damage as shown.
The helmet has some damage to the dome, and has a full liner and chinstrap as shown. The liner is very salty but is still intact. The chinstrap is a bit stiff.
The 9th Infantry Regiment saw extensive combat during World War I, namely in the Aisne, Aisne-Marne, St. Mihiel, and Meuse-Argonne Offensives, and heavy defensive sector combat in several sectors.