To be fair, 'tallica have been using ESP since day dot
Really, so the Gibsons, Fernandes, Jacksons they used for Kill 'em all, Ride the lightning, Master of Puppets and possibly Garage days revisited were a figment of my imagination. I believe they started endorsing ESP just before the Justice for all album as Hetfield was looking for consistent sounding guitars as his Gibsons all sounded different to each other. Or for more money, depending on you point of view.
So, hardly year dot, unless you count their first three albums as not worthy of classing as Metallica?