This is Templar style helmet from the 3rd to 5th Crusades of around 1200 AD, flat topped with tapered sides, chain mail coif and nose guard but normally seen with a full face plate.
Made by White Rose Armouries as bespoke piece, based on image in a Osprey Man at Arms series book, I think it was the Knights of Outremer, but no longer have the book to confirm that.
The image above is from Bolsover Castle and the one below from Warwick from the early 90's, during a period I did medieval style entertainment shows for the public, which skirted the edge of medieval authenticity.
I did have long hair and big beard back then.
I still have this helmet.
I even have a cartoon image created for me wearing the helment
