What causes green staining on the front rim ?

Verdigris. Usually on brass / copper in a dry hard form. I have seen this type of build up before where there is a reaction to the brass near leather. Sometimes inside brass fitted leather pouches the internal brass rivets can have a kind of soft waxy verdigris build up that can be moved way with a finger nail.
 
In the past – older owners of the helmet often used a brass cleaner called Brasso. they learn this from the American Army. So you had a short time shiny brass and a longtime culprit. What is worse is that the Mercury gilding also disappeared with his compound. The gilding gave rise to the phrase
"mad as a hatter" because of the fumes caused by the Mercury when gilding. I understand this technique has been started again in India. All hearsay.
 
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