Rich - thanks, that's the paper I referred to. Note date -- 6-Jan-80 - the work being done in the summer of '79.  A number of us had been using ditroff as it had existed for a couple of years by that time, and as the abstract says - they got the Mergenthaler in the Summer of '79.  What I did not remember was when the Mergenthaler work started.  I did remember that it was after the APS-5 had been available for a while.

Just for completeness, Judge Green mandated the breakup of the Bell System on January 8, 1982 -- Tool Chest et al. is set up later  --  all part of the ability for AT&T to be in the commercial computer business.