That's a Roman helmet made of iron, brass and copper, fabricated sometime around 125 to 175 A.D.
Fantastic condition aside, the helmet is missing two important parts: The cheek-guards. They either got lost sometime over the past 1,874 years, or maybe someone left them in the back of an Uber. But master armourer Jeffrey Hildebrandt, who recreates historically accurate armor and runs Royal Oak Armoury in Canada, took his best guess at what they'd look like. Hildebrandt recreated the entire helmet from scratch, and the finished product is stunning:
"A conjectural cheek piece," Hildebrandt writes, "conflated from several indirect sources."
Lastly, Hildebrandt took a little license with the neck protection:
"An unusual feature of the original helmet is a copper-alloy flange inside the neck guard bearing three slots," he writes. "The slots appeared to have been for articulating additional plates to, for the purpose of extending the protection of the neck. The two lower plates here are conjectural."
If you want to LARP with one of these on, good luck; the helmet doesn't appear to be for sale, and Hildebrandt's current work backlog is 24 to 36 months. (Then again, if you've waited for 1,800 years, what's a few more?)