The second edition of Unix was developed for the PDP-11 at Bell Labs by Ken Thompson, Dennis Ritchie and others. It extended the First Edition with more system calls and more commands. This edition also saw the beginning of the C language, which was used to write some of the commands.
The code in the Unix Archive is only the source to some of the commands, some of the library functions, and the C compiler. The files in c/ come from the last1120c.tar.gz tape, and form a working C compiler for Second Edition Unix.
The files in lib/ come from the libc.sa file which is on the last1120c.tar.gz tape, and form the C library for the compiler.
The files in cmd/ are recreated from the text fragments found on the file s1-bits.gz. These were reconstructed by Doug Merritt and Warren Toomey, and form the source code to some of the commands in Second Edition Unix.
For more information about Second Edition Unix, see The Evolution of the Unix Time-sharing System by Dennis Ritchie.