I’ve never found any reference material to confirm/deny.

It is incredibly hard to find mid-70s technical documentation on weird niche software like TECO Emacs. I mean, all of Emacs’s founders are still around as far as I know, so there might be a chance to find someone on the GNU Emacs mailing lists who knows how to reach out to them. While early Unix history is preserved rather well (thanks to Doug McIlroy and Dennis Ritchie who kept quite an archive, it seems), pre-Unix history predates public interest.

I have never asked myself when exactly the Emacsen had grown said capability. Now I’m intrigued myself!