If I recall correctly (and it's been a while...) csup, CVSup, et al
were based on `sup`, which was a file distribution tool somewhat like
`rdist`, which came from CMU. csup/cvsup was optimized for moving
source code deltas (a la CVS repositories) around. I recall a
graphical client written in Modula-3?
Right -- ??maybe?? Bob Baron or one of the Mach guys did it - used the DEC Modula-3 IIRC.
The SUPDUP protocol used by ITS hosts was a "Display Protocol" based
on TELNET: https://www.rfc-editor.org/rfc/rfc734
Exactly - popular with the LISP hackers. I started to implement it in the CMU Distributed Front-End years ago but never finished it - I don't know if it was ever completed.