In early 1968 Iron Butterfly released their first album - HEAVY - (material written in 1967). The only song on it that I'd classify as close to heavy rock or metal was the song titled "Iron Butterfly Theme" and truth be told it's a bit more Trippy than Metal...
So it looks like Iron Butterfly entered the heavy metal scene with the song "In-a-Garda-Da-Vida" which was released on their second album in 1968. However, there's this interesting little quote from wiki...
" Jeff Beck claims that when he saw Iron Butterfly perform at the Galaxy Club in April 1967, half a year before the band recorded their first album, their entire second set consisted of a 35-minute long version of "In-A-Gadda-Da-Vida".
So in light of that early 1967 performance (which might not have even been their first performance of the song) we might very well have the seed that started the heavy metal movement...
By contrast, at roughly the same time The Beatles were singing this song...
I got to see the "original" lineup of Ingle, Brann, Dorman and Bushy in the late 80s at an outdoor concert at the Miami Metrozoo(!). Interesting crowd, great show-although there was a strange smell in the air
. Side observations re the video: I don't think this is what "The Ventures" had in mind for 'Mosrite' guitars and basses. Also, psychedelic video effects suck in black and white.