Jack's right about the 35th Infantry. It has what's left of the correct WW1 liner, so I'm sure it's not a 1917A1 (WW2 era) helmet. It looks right to me, there should be a stamped makers code in it, would you post that here? That will tell us if it's an American made helmet or from one of the many other countries who manufactured them. Not being an American made helmet won't disqualify it from being original though, they were used by several countries during the war.
The 35th saw some pretty serious action, in that condition I'd expect it to bring maybe a couple hundred $