“Franco-British modern armour to be worn under a tunic and British splinter goggles MK I…” – Imperial War Museum.

The source does not list the World War I armor materials or any other information. When I think ‘armor’, though, I think metal. It also looks metallic. I wonder if it was used in combat?

Sources: Both images are by Horace Nicholls. Credit Imperial War Museum, public domain. Front View, and back view.