If you consider what I've learned of the helmet from the previous owner, in conjunction with what Art Andrews and FettPride have contributed, I feel there is absolutely no way this is a Sgt Fang Mystery Helmet recast. First, it preceeds the existence of the Fang. It's made of fiberglass, as opposed to the Fang which is some slush resin. Moreover, this came out of Don Post Studios. However, the Fang is more a full shape whereas the Don Post Deluxe might have been made smaller through nipping and tucking in order to appeal to the child-sized market. Moreover, the scars it has are identical with those in the mass production line. It makes no sense for someone to add them to a Sgt Fang so identically as these marks did not exist on the screen-used and are of no value.
This helmet came from the master mold used by Don Post Studios but wasn't part of the production, and LFL Marketing later sent it to the artist who used it as a reference for a Fett action figure he sculpted in the 90s.
Structurally, there are significant differences between the Don Post helmets and the Sgt Fang. The Fang has a fuller look to it whereas the Don Post Deluxes (fiberglass) and other DP helmets have a more blocky, parallel look to it.
Just pretend for a moment that you had a full size helmet made out of paper or aluminum foil. If you were to cut a certain amount out of the ear platforms and push the newly cut edges in, then apply bondo to re-establish the surfaces, you'd basically get a narrower helmet that otherwise is still roughly 1:1 scale.
Hope this makes sense. The lineage chart seems to suggest the Fang came from a lava helmet or some plug that had the blocky shape of the Don Post. If that was indeed the Fang's progenitor, then Fang did a pretty amazing job in building it out to a more full and rounded shape. If that's not what he did then the lineage chart might be a touch inaccurate. At any rate, I keep going over in my head what kind of mods this helmet ( or a casting from it) would have to go through to rebuild that full form and shape of the movie helmet, as it would be an incredibly fun and fascinating research project - artistically and mentally stimulating!