By Stephen A. Cook. Proceedings of the Third Annual ACM Symposium on Theory of Computing, May 1971, pp. 151–158. DOI: doi.org/10.1145/800157.805047
Author publication list: www.cs.toronto.edu/~sacook/
Author-hosted original scan: www.cs.toronto.edu/~sacook/homepage/1971.pdf
Revision-3 transcription: www.inf.unibz.it/~calvanese/teaching/tc/material/cook-1971-NP-completeness-of-SAT.pdf
The transcription’s first-page footnote credits Tim Rohlfs as transcriber. Neither a transcription date nor original authorship by the transcriber is inferred. Free University of Bozen-Bolzano hosts this copy; ACM publishes the original proceedings.