Bob Swartz, founder of Mark Williams Co, has allowed the sources for COHERENT to be published under a three-clause BSD license.  Steve Ness is hosting them.  They are available here:

    http://nesssoftware.com/home/mwc/source.php

For reference, for folks who don't know what COHERENT is, it started as a clone of 7th Edition, but grew more modern features over time.  Dennis Ritchie's recollections of his interaction with it: https://groups.google.com/forum/#!msg/alt.folklore.computers/_ZaYeY46eb4/5B41Uym6d4QJ

And of course the requisite Wikipedia link: http://en.wikipedia.org/wiki/Coherent_(operating_system)

        - Dan C.

PS: I hold a soft spot for COHERENT in my heart.  I became interested in Unix in high school, but this was before Linux was really a thing and access to other systems was still hard to come by.  I spotted an ad for COHERENT in the back of one of the PC-oriented publications at the time, "Computer Shopper" or some such, and realized that it was *almost* within my reach financially and that I could install it on the computer I already owned.  Over the next month or so, I scraped up enough money to buy a copy, did so, and put it on my PC.  It was quirky compared to actual Unix distributions, but it was enough to give one a flavor for things.  The manual, in particular, did not follow the traditional Unix format, but rather was an alphabetical "lexicon" of commands, system calls and functions and was (I've always thought) particularly well done.  Links to the COHERENT lexicon and various other documents: http://www.nesssoftware.com/home/mwc/.