Thanks for sending the mystery disk, it looks like it might be the cracked and modified version by sarnau that he described here. I've uploaded the disk image to that thread as Atariage wouldn't let me attach it.
It's on a single disk, with individual files for sprites and level data, a medium rez level editor (that modifies a regular Gauntlet 'Disk 2' not the files on this mystery disk), and most interestingly of all MIDI.PRG which runs but doesn't seem to do anything- perhaps it is translating joystick input into MIDI messages though!?
The version of Gauntlet appears to be subtly different, at least the palette has been tweaked and the info text moved around a bit, here are some comparisons with 'normal' versions Gauntlet on the left and the 'mystery' disk on the right:
Hopefully someone can try it on two midi-connected STs and see if the MIDI code works, it's probably been a while since anyone has seen 4-player Gauntlet. Would be nice to compare the scrolling too to see if it has the blitter code described by sarnau.
Edited by galax, Mon Feb 1, 2016 1:17 PM.