Thanks Jim and Alan for the interesting information, and links to these nice documented great helms.
What stroke me first
was the dark, brownish black metal color of my great helm, which is very similar to the color of the documented 14th century English great helms, such as the one of Sir Richard Pembridge: there is some light corrosion to the metal with a sort of surface coating of hard black oxide. In my opinion, this should be noted as most helmets from the period are often seen 'bright' metal.