On page 4 of Kenneth H. Rosen, Richard R. Rosinski, James M. Farber, and Douglas A. Host, UNIX System V Release 4: An Introduction, 2nd Edition, the subsection titled "Open Source Code" has the following first two sentences:
"The source code for the UNIX System, and not just the executable code, has been made available to users and programmers. Because of this, many people have been able to adapt the UNIX System in different ways. This openness has led to the introduction of a wide range of new features and versions customized to meet special needs."
The book by Rosen et al. cited above is has year of copyright 1996. There is apparently an earlier edition from 1990. This is no ordinary book by obscure authors--it was considered as one of the "bibles" for its subject at its time and would have been familiar to many. Already in the above description there are the crucial concepts of the importance of source code availability and adaptability.