Not so fast. The guitar chords in In-Agadda-Da-Vida were laid down before Priest was even a band. If you are talking theatrics & looks, then it probably was Priest. Iron Butterfly probably considered themselves just Rock. This brings me to a point of clarification needed. Is heavy metal considered the structural arrangement of a song or it is an all up music, attitude and look combined?

I would say it would be the later. Case in point, The Kinks were the first to cut their speakers to produce fuzz. Is fuzz considered HM or are just power cords considered HM? The Kinks are not HM.