Hopefully below are some pics of 2 US experimental helmets.
The first being a US manufactured model 1917 helmet which has the experimental german style liner. Some were freshly manufactured and others were simply converted from captured stock. In addition to the leather 3 pad liner it has a foam insert instead of a rubber doughnut.
The second helmet is a US No.5 experimental helmet. These were manufactured by Hale and Kirkburn Co (Philadelphia) during late 1917. Only about 2000 were made and are one of the few experimental helmets that were actually trialed in France (supposedly). Its chin strap is dated 7-18 (July 1918). The pics haven't picked up the camo paint finish over the original drab paint.
I have many other egs of experimental helmets from ww1 and will post other pics if anyone is interested?