Nah, doesn't seem like it would be an MS. I'm still stuck on the size and shape of the dome. . .
It rises pretty much vertically right in the front where the arrows are.
Going by the pics, the left ear seems slightly warped. The narrow cutout in the center of the ear
seems to have a gentle curve in it.
Looks noticeably larger than the MR cast, too. There aren't too many helmets by "known makers"
that are that big. The MSH2 is the biggest one I can think of, but that dome is really throwing me off.
Could be a one-off fan made helmet I suppose?