Dave Mustaine brought this song to the band and worked on it with James Hetfield and Lars Ulrich. At the time, they called it "The Mechanix," and included it on their 1982 demo cassette No Life 'Til Leather
After Mustaine was fired from the band in 1983, Metallica released Kill 'Em All
with a re-worked version of this song renamed "The Four Horsemen." Mustaine's new group, Megadeth, released it as "Mechanix" as the last track on their first album, Killing Is My Business....
Mustaine's version has completely different lyrics, but the music is very similar. Since the Metallica song had been out for a while, Mustaine would often introduce "Mechanix" in concert by explaining that it was a track he wrote with Metallica.
Terry - Belleville, Canada