The 'second' C compiler was a PDP-10 and Honeywell (36-bit) target Alan Synder did for his MIT Thesis.

It was originally targeted to ITS for the PDP-10, but it ran on Tops-20 also.

My >>memory<< is he used a 7-bit Character, ala SAIL, with 5 chars stored in a word with a bit leftover.

You can check it out:  https://github.com/PDP-10/Snyder-C-compiler

I believe that C compiler Nelson is talking about I believe is actually Synder's that Jay either ported from ITS  or WAITS.

We had some form of the Synder compiler on the PDP-10's at CMU in the late 1970s.

It was either Mike Accetta or Fil Aleva that wrote a program to read PDP-10 backup tapes, that I updated to deal with TOPS-20/TENEX 'dumper' format which was similar/only different.

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On Thu, Jul 15, 2021 at 3:03 PM Norman Wilson <norman@oclsc.org> wrote:

Nelson H. F. Beebe:

  P.S. Jay was the first to get Steve Johnson's Portable C Compiler,
  pcc, to run on the 36-bit PDP-10, and once we had pcc, we began the
  move from writing utilities in Pascal and PDP-10 assembly language to
  doing them in C.

======

How did that C implementation handle ASCII text on the DEC-10?
Were it a from-scratch UNIX port it might make sense to store
four eight- or nine-bit bytes to a word, but if (as I sense it
was) it was C running on TOPS-10 or TOPS-20, it would have had
to work comfortably with DEC's convention of five 7-bit characters
(plus a spare bit used by some programs as a flag).

Norman Wilson
Toronto ON