Welcome to our liguistic cauldron, Mike.
I recently had this discussion with someone 15yrs my junior about what constitutes the style of "heavy metal". Since, as you know, it has been around so long and undergone a number of media driven definition changes, I found the closest band that we could agree that fits the label was Metalica. It's good to hear that actually® fits the *original usage requirements, however, she was under the impression that Metalica is the 'definition' of heavy metal. If one thinks about it as describing the thickness and musical weight of the sounds used, well she's correct... and it is, indeed, even heavier than Led Zeppelin - which I understood (as you described) was one of the first to which the term was applied. She has excellent aural capacity and quickly pointed out Zeppelins "folk music"(as you and I might call it) flavor which she claimed didn't make it metalic enough.
To paraphrase an analogy I made to her- "Just because we kept finding new/more complex molecular structures that fit science's definition of metal, it didn't make the more *simple metals any less 'metalic'.
