​There was a PDP-10 C compiler in the late 1970s, that was kicking around CMU, MIT and Stanford which we used to write backup10 and ​and an implementation of tar.   IIRC, it was based on the the Ritchie front end and was V6 in syntax (i.e. pre-V7 or typesetter C - aka 'White Book).   I've forgotten the rules of chars, but I remember you had to be careful.   I think it was 4 9-bit chars to transfer things (4*9=36 bits), but I think I remember there were cases on output that it wanted to wash it through a 7-bit PDP-10 char (5*7+1 =36bits) which was the 'norm' for most languages like SAIL, BLISS et al.

I did not mess with much, but that time, I was transitioning from the 10's to UNIX by that time.   I added support for the -20's dumper tapes to backup10 which were almost but not quite the same.  But that was the last I messed with it.   Mike Accetta and Fil Aliva (of CMU Mach fame) I remember had their had in that subsystem, at one point.  And of course Danny Klein is always a good one from those days to ask too. I'll see if I can dig them up and ask.