I believe this to be a Korean War helmet. Note the black leather chin strap found only on MP helmets.
This helmet has surface rust and paint wear as shown in the pictures. The underside has surface rust spots mostly on the walls and some in the dome, along the rim and spots on the outer surface. The pictures are a good source of that information. Rear seam and swivel bale. Some minor depressions on shell.
The Mine Safety Appliances liner is crack free with chipping, white paint. There is a 30 above the very readable dome stamp. The liner has the black leather for MPs, including the liner chinstrap with brass clamp. The hole in the liner front is filled with white paint.
The suspension system is in very good condition with no tears. The helmet does not have much texture left. Most of the texture is due to the paint wear.
The item "US MP KOREAN WAR HELMET REAR SEAM SWIVEL BALE WWII MINE SAFETY APPLIANCES LINER" is in sale since Wednesday, August 09, 2017. This item is in the category "Collectibles\Militaria\WW II (1939-45)\Original Period Items\United States\Hats & Helmets". The seller is "nrscss" and is located in Glenmont, New York. This item can be shipped worldwide.