I know it's not exactly metal, but **** it, this deserves to be in this thread.
It's amazing how it didn't even take a year for Black Sabbath to begin heavily influencing bands. I've been on a kick of digging through "proto-metal"/heavy psyche stuff from the very late 60's and early 70's. So far, the earliest band that was definitively heavy and didn't have any residual psychedelia in their sound is Iron Claw. This song, Skullcrusher was recorded by them in 1970(!), and it wasn't released until the late 2000's as part of a compilation of the 15 or so songs Iron Claw recorded over 5 years until 1975.
One of the band members has youtube videos of Iron Claw playing War Pigs and Fairies Wear Boots two months before Paranoid was released. Apparently, the guys in Iron Claw went to Sabbath shows and recorded the concerts and then went back and just learned the songs by tape. You can even hear them singing the original lyrics to War Pigs. I'll throw those versions in the spoiler.