Nice job -- one comment: You said at the end of your document:
"Both cno and phx were written at what appears to have been what I would call v6b, version 6 as the standard library was moving from libS to libc, as Dennis Ritchie formalised the library proper. This can be seen in crt0.s where the form is not of standard V6 but of the libc V6 before it became standard in V7 and changed yet again. I was able to track this using the UNSW archives which have both libraries at the critical point."
I will place a bet it is not your v6b idea... (which was basically V6 plus Ken's patch tape - although as Noel and I have decoded some of PWB 1.0 - which was based on V6 too -- made it into the wild in a couple of places). Anyway, I think you are seeing code output from what was called the 'Typesetter C" compiler release which came out before V7 and was needed to compile troff et al. which actually what conforms with the original K&R. Indeed, that compiler used libS as the library.
The other thing, many v6 implementations (like CMU's and I think MIT) had a preprocessor (it was called /lib/cpp ) before we got Typesetter C. i.e. a version of cpp that predated the Riesner cpp rewrite that was in Typesetter C and later V7. Steve Johnson probably knows the history of the preprocessor better than I. I know when we went through the V5 to V6 upgrade we somehow had a preprocessor. I remember being curious about it and then being disappointed it was not as good as the BLISS macro facility.
But ... neither the binary nor the sources for that cpp are in Warner's Archives for V6 when I just looked.
The Interdata 7/32 stuff Warren has is a V6 port, and has a version of cpp, but a note in the source says it is based on what looks like the Typesetter C compiler - which was sometimes called the Level 7 compiler because Dennis had told us all it was the new compiler for the next releases (TS and V7).
PWB 1.0 has a copy of cpp in the sources, but a quick look its not clear how close the V7 version it is.
So, unless anyone else can illuminate, I'm not sure where the first cpp that some of us using v6 had originated.