The 'ken-sky' tape had some binaries on it before it was overwritten. There
also seems to be some unknown data halfway through the free blocks that I
couldn't recognise.

I have recovered the binaries and identified all of them. These bins fall
somewhere between V3 and V4. The directory structure and other elements match
the V3 manual, as do the syscalls (e.g., the arguments for kill(2) differ
between V3 and V4, and these bins use the V3 arguments). The features,
however, are closer to V4. For example, nm(1) had already been rewritten in C
and matches the V4 manual's description. The assembler also matches the V4
manual in terms of the number of temp files, and the C compiler refers to the
assembler as 'nas'. The assembler is located physically between files starting
with "n" and "o," and the files around it follow a weak alphabetical order, so
it is logical to assume that it was named "nas".