Having been at this show and having seen the trio many times in the '70s, I'd say that while this music is great, it's not the definitive document of the working group as is The Quest from the trio's heyday (recorded 1976, released on Red in Italy and Pausa in USA). There's one other recording by this trio, similarly hard to find if not more so, but I'm blanking out on the title.
Ah, found it here:
http://www.bb10k.com/RIVERS.disc.html
Paragon, from 1977. Definitely more obscure than The Quest.
Listening to The Quest right now. Exquisite. If you can find a rip on the internets, grab it.