[url]http://youtu.be/fcGKBUFD9qU[/url]
So far, from what I've gathered from other fans on the 343i site, it's closest to the venator armor though not the same, stirring questions as to why it's there and the possibilities behind it. Enjoy.
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Look like a rendering bug. Most likely its just a regular helmet that was for some reason rendered at a lower LOD