You would absolutely normally get source to the vast majority of the unix components with a unix install.

At a bare minimum, you would almost have to have source to the vast majority of the kernel because there was no kernel modules. Sysgenning for a particular system involved compiling the kernel for that set of hardware.

I'm literally working on an early sun system emulator right now, heavily assisted by the standard sec tape that came with the standard software distribution of sunos, which has plenty of AT&T code.