From: Warren Toomey
The second part of:
"Sixth Edition sources are much more widely available than earlier versions,
thanks largely to the Lions book"
is I believe incorrect, though; I reckon it's because V6 was so widely
distributed, both inside and outside Bell. Many more copies -> higher
probability of retention.
Also, from 'Source code listing for the Lions' Commentary in PDF and
"in 1988 I discovered an old 9-track tape being discarded of a PDP11
backup. It was hard to determine what it was running, but it did have an
intact /usr/src/ tree of which most of the files were timesamped 1979, even
at that time it seemed ancient. So it was either 7th edition or a derivative
like PWB, which I believe it was.
Do you still have the tape, or its contents?