There was a track of USENIX 1986 called "UNIX on Big Iron."  Peter Capek of IBM was the chair and Gene Miya and Jim Lipkis rounded out the program committee.  The proceedings are available.

Program included:
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nygeek.net
mindthegapdialogs.com/home


On Mon, Dec 19, 2022 at 12:38 PM Phil Budne <phil@ultimate.com> wrote:
The October 1984 BSTJ article by Felton, Miller and Milner
https://www.bell-labs.com/usr/dmr/www/otherports/ibm.pdf

Describes an AT&T port of UNIX to System/370 using TSS/370
underpinnings as the "Resident System Supervisor" and used as the 5ESS
switching system development environment.

I also found mention at http://www.columbia.edu/~rh120/ch106.x09
chapter 9 of http://www.columbia.edu/~rh120/ with footnote 96:

      Ian Johnstone, who had been the tutor at University of New
      South Wales working with Professor John Lions, was one of the
      researchers invited to Bell Labs. He managed the completion at
      AT&T Bell Labs of the port of Unix to the IBM 370 computer. See
      "Unix on Big Iron" by Ian Johnstone and Steve Rosenthal, UNIX
      Review, October, 1984, p. 26. Johnstone also led the group that did
      the port to the AT&T 2B20A multiprocessor system.

I found
https://ia902801.us.archive.org/3/items/Unix_Review_1984_Oct.pdf/Unix_Review_1984_Oct.pdf
"BIG UNIX: The Whys and Wherefores" (pdf p.24), which only offers rationale.

Also:

        "IBM's own involvement in Unix can be dated to 1979, when it
        assisted Bell Labs in doing its own Unix port to the 370 (to
        be used as a build host for the 5ESS switch's software). In
        the process, IBM made modifications to the TSS/370 hypervisor
        to better support Unix.[12]"
at https://en.wikipedia.org/wiki/IBM_AIX#cite_ref-att-s370-unix_12-0

Is there any other surviving documentation about the system?
Any recall of what branch of AT&T UNIX it was based on?

Thanks!
Phil