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 I had no exposure to ITS and so have no knowledge of its history
 or the tools that it offered.  Was TECO truly its first editor?

Yes.  PDP-10 TECO came before ITS.

The PDP-10 was an upward-compatible improvement of the PDP-6.
The AI lab first got a PDP-6 and wrote a TECO for it.

The PDP-6 was not suitable for timesharing.  So when it got the
PDP-10, people immediately used TECO on that too, but they also wrote
ITS.