Comment by inkyoto

AT&T did not ship with the kernel source code, but they often shipped with the compiled object files of the kernel and a command line utility that allowed to change the kernel configuration parameters, after which the kernel would get re-linked into a new one.

Not open source by any definition, but it was a viable way to obtain a new kernel image. The practice has become obsolete after the adoption of loadable kernel modules across nearly all UNIX flavours, with the exception being OpenBSD (if my memory serves me well).