A WWI "Ersatz Helmet" - a Metal Pikelhaube.
 

In 1914 German Pikelhaube began to be produced from materials such as felt, steel, tin and vulcanised fibre due to leather shortages.
Originally painted black with bright or painted or chemically dulled helmet plate, spike and chinstrap fittings, they were later to be overpainted field grey.
Instructions were issued for spikes to be removed, in some cases they were easily removed, the spike being unlatched from its base, other spikes were held on by domed split pins, others, like this example were fixed to the skull by flanges and so had to have the flanges bent or broken or be knocked off (the spike on this helmet is a n original flanged one that isn't affixed).
The ersatz helmets were relegated to rear line service as leather pikelhaube and then the stahlhelm became available.

This example bears an extremely thin brass Prussian eagle helmet plate secured to the helmet by means of a very small brass nut and bolt.
I'm not well versed in these helmets but this is the only plate with this type of fixing that I've ever seen or heard of.

The second image shows what happens when a friend accidentally drops an old (inert) Mills bomb on such a helmet...........

Ohhh!!! You had been sharing your bourbon collection with him when the mishap occurred?

Er, yes, now you mention it........
Some of the survivors and some of the offending fluids: