Thinking about this typesetter C may have been later with ditroff.  

No.

What I remember, there was first roff written in assembler.  It was

then rewrittn into C (but now with real macro capabilities.  roff

has a lt build in), Typesetter C appeared in Edition 6.1 (of 6.2)

since the n/troff code demanded a lot from the C-compiler.  To make

it possible to run it on a pdp11 there was the hack turning data

into test (for the hyphenation tables).  Ditroff was done by bwk

to be devie independent.

Regards,

jaap