What innovative product design feature was introduced in the M1 helmet that was NOT present in its WWI predecessor, the M1917 'Doughboy' helmet?

The iconic M1 helmet was a revolutionary piece of military equipment that protected American soldiers across multiple conflicts. Its design represented a significant advancement in combat protection technology and became one of the most recognizable symbols of the American military. This poll tests your knowledge about the product design evolution of this critical piece of military equipment that influenced soldier safety for decades.

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A two-piece construction with separate steel shell and liner
Use of manganese steel instead of standard carbon steel
A chin strap attachment system
The iconic olive drab green color