Reused NS64 M40 Stahlhelm by fire brigade.

[ShapedSteel]

1 thing about the liner and treating it , it won't be gone in 3 years without treatment. I have much worse liners.
The fact just is that these products are not made for conservation of leather but to keep leather useable (supple) and works best with leather that's not almost 100 years old :-)
None of my leathers which I own for +15 years have noticeable deterioration. Also all the leathers treated very long ago (60's / 70's) have not improved at all as compared to none treated leather.

So my golden advice , keep chemical products away from your helmets.

Thank you very much for the advice!
In case I ever dig up a helmet with liner, would applying anything to it be recommended? I've heard that after helmets have been dug up the leather shrinks.