Medieval and Historical Firearms Know-it-All Incoming
If I had to guess, I would say that the helmet is probably simply hardened leather. Of course, I'm drawing from historical knowledge and I don't believe there was a single historical civilization to use all-leather helmets (or armor pieces on their own for that matter), but generally leather was by itself (within a single garment, I mean; very few accounts of leather armor by itself). Correct me if I'm wrong.
It would certainly be pretty protective if it were really layered on just that thick. Boiled leather is pretty tough.
Also, I'm not entirely sure you can really say it would stop firearm projectiles. Buffalo are resistant to bullets (both modern and historical) because of their tough, muscular frames and thick bones. A bit of their skin might not stop a bullet on its own.
Then again, is this cow leather or some mystical Zelda creature leather?
And also, on the note of the 'Partial' armor, that was indeed a thing. For a long time, pre-professional army raising and even a good ways into those times, armor had to be bought by the individual wearing them unless their master equipped them. A really good example of this would be the gambeson cloth armor used in the Hundred Years War. Most of the time it would simply be layer upon layer of cloth, sometimes interspersed with tiiinnyyy bits of metal along places where they expected to be slashed.