While I won't go so far as to say I'm a "leading authority," I can add that I'm 44-years old and have collected helmets for 32 of those years. I have a personal library of thousands of books, and here is a link to my helmet book library:http://www.nyc-techwriters.com/militari ... titles.htm
I have just about every book ever published on military helmets. I also have three dozen binders full of articles published on military helmets (not including my own) dating back to the 1970s. I attend major militaria shows around the world, and I know all of the major living authors of helmet related matter. What every serious helmet collector agrees upon is that there is no class to teach this stuff, no course of studies, and instead we learn through study. Stuart Bates easily has the largest collection of Wolseley sun helmets in the world and thus has seen more of these up close and personal than anyone.
I have focused on colonial pattern sun helmets from around the world, while also collecting steel, ballistic, tanker, and even some aircrew helmets. With sun helmets I have British, American, French, German, Austrian, Italian, Spanish, Portuguese, Canadian, Argentine, Australilan, Vietnamese, Ethiopian, Beligan, Dutch, South Africa, etc., etc.
One other area of study of mine has been experimental helmets and I can tell you I've seen a lot of experimental steel helmets and Stuart Bates and I have owned a piece now in his collection (we traded), which is a rare experimental British helmet. So we've had experience with experimental pieces.
Additionally, I have seen examples of military helmets in museums around the world, many of which I have posted photos from on one of my websites: http://www.firearmstruth.com/featuresmuseum
Finally I will say this... there is HUGE money in fakes in militaria. People get burned all the time. The fakers are getting better all the time, and it includes helmets, uniforms and just about everything else. Throw in movie props, theatrical pieces, helmets used by secret socities and fraternal groups and there is confusion. But just because you want something to be real doesn't make it so.