My understanding is he had at least one custom one made. It looks like a Mark but it had a big knob in the middle IIRC. It's popped up in the background of a couple studio vids of Metallica and I remember a couple years ago there was a video with Hetfield talking with a wall of amps behind him, and it was sitting in the background.
Either way, Flemming Rasmussen has published all the notes on the AJFA tones, so the cat has been out of the bag for a while. Plus with AI now you can separate guitar tracks on mastered albums and use even a $80 Mooer pedal let alone a Kemper or Axe FX to tone match it. There's a bunch of free IR packs too that will get you there as well. Plus it's less of a secret now how to get a great metal tone out of an early Boogie Mark amp, as the YouTube information revolution the past 2 decades has really killed the gatekeeping that was so prevalent for so long in the music mixing business.