I did go back for it but it had gone.
That's an interesting thing, Jimpan. I have not seen that anywhere before. Do you have any idea how many of those were produced, and when?
Actually I have no idea what's the deal with these helmets. I have only seen one before mine at this page about swedish helmet.
Militära huvudbonader använda i Sverige - Hjälmar
He does not know for sure either but same guess as me, experimental.