We missed this earlier this year, but Coherent has been released as open source. Coherent is a UNIX clone originally developed for the PDP-11, but later ported to a number of other platforms, including the IBM PC. It was developed by the Mark Williams Company, and despite an official investigation by AT&T, no signs of copied code were ever found.
Mark Williams Company closed in 1995. In 2001, Bob Swartz asked me to archive the hard disks containing the Mark Williams source repository; the command and system sources here are from that repository. I have long intended to catalog and organize these sources, but in the meantime they are posted here as is. MWC's documentation guru Fred Butzen provided the MWC documentation sources.