The Sixth Edition of Unix was released by Bell Laboratories in May 1975.
It was designed to run on the PDP-11s with 18-bit addressing such as the PDP-11/40, 11/45 and 11/70. The kernel's image size was around 29 Kbytes. This edition introduced the `Portable C Library' written by Mike Lesk, which would evolve into the Standard I/O library.
For more information about Sixth Edition Unix, see The Unix Time-sharing System: A Retrospective by Dennis Ritchie.