Matt G wrote:
> C is also available on the HIS 6070 computer at Murray Hill, using a
> compiler written by A. Snyder and currently maintained by S. C. Johnson

Some of which has been preserved.  E.g. search for HIS-6000 in these files:
https://github.com/PDP-10/its/tree/master/doc/c

Here's some definitions for the code generator:
https://github.com/PDP-10/its/blob/master/src/c/h6000.gt