The Mystery Helmet is a generic label applied a group of helmets most likely developed by Don Post Studios that are derivative and modified castings of a preproduction helmet.
They were labeled Mystery Helmets at the time, because the helmet's origins were unknown. The name is somewhat of a misnomer now that research has produced a partial origin.
I know of at least five different Mystery Helmet versions, three of which are like the Sarge Fang and Marrow Sun helmet. The other two Mystery Helmets represent earlier castings and have a few less modifications.