In an article on the Guitar World Magazine site covering the 30th anniversary of Metallica's seminal album ...And Justice for All, ESP player Kirk Hammett was quoted from a 2002 interview where he discussed in detail the process and gear that went into recording the iconic song "One". The album was released on August 25, 1988.
Kirk talks about working on the song's extraordinary solos. "The first solo was a little bit more worked out. I heard James playing some really melodic stuff over the intro, just doodling around, and I thought, 'That's pretty cool, I'm gonna use part of that.' So I have to give credit to James for subliminally pushing me in that melodic direction," he says.
In the interview, there's also a lot of discussion of the gear used at that session.
"I think at that point I was using the ESP neck-thru-body KH-1 guitar, with the skulls on the fingerboard. I'd gotten that guitar in '88 and used it pretty prominently in the studio. I also used an ADA preamp and an ADA MP-1—it was a programmable digital amp that had tubes in it, with a separate rack-mounted EQ. I remember blending that thing with the Boogies for lead sounds and clean sounds. The clean sound on 'One' was done almost exclusively with the ADA MP-1."
Even though Kirk refers to this guitar as a KH-1, he is almost certainly referring to his original neck-thru KH-2 model. The KH-1 was a V-shaped model, while the design of the KH-2 was based on the ESP M-II (or Mirage) body style. Also, the KH-2 was the first model to use Kirk's skull-and-crossbone fingerboard inlay design. Mystery solved. But just to confuse things a little more, Kirk was shown playing one of his ESP Vintage Plus guitars in the now-famous 1988 video for "One" that shows the band's performance interspersed with scenes from the 1971 anti-war film Johnny Got His Gun, which was the inspiration for the song.