It’s a port of 4.3BSD Tahoe to the i386, utilizing the Mach kernel. Well I didn’t install the X11 stuff as I’m just not in the mood to fight it, but it’s a 4.3BSD system! What is really interesting is the other disk set, MtXinu-Mach386-M3921131020-Mach_3.0+DUI.7z which is that Mach 3.0 kernel version MK78 which I believe is the first widespread & public release of Mach 3.0. Being that this a Mach based system it builds the 3.0 kernel with ease. It even includes a 4.3BSD (sadly binary only) ported kernel to the 3.0 Mach which you can run.