At the end of 2015, Doug McIlroy sent this e-mail in to the TUHS
mailing list:

  Among the papers of the late Bob Morris I have found a
  Unix manual that I don't remember at all--a draft by
  Dennis Ritchie, in the style of (but not designated as)
  a technical report with numbered sections and subsections.
  It does not resemble the familiar layout of the numbered
  editions. Besides the usual overview of kernel and shell,
  it describes system calls and some commands, in a layout
  unrelated to the familiar man-page style. Detailed
  reference/tutorial manuals for as, roff, db and ed
  are included as appendices.
   
  The famous and well-justified claim that "UNIX contains a numer
  of features very seldom offered even by larger systems"
  appears on page 1.

  The document is evidently ancestral to both the recognized manuals and
  the SIGOPS/CACM paper. It apparently dates from mid-1971 when Unix had
  been running for a "few months" on the PDP-11. At that time there were
  only 21 system calls, a number that had increased to 34 by November
  when the v1 manual was produced.

A PDF scan of this draft manual is available here as the file
UnixEditionZero.pdf.