Not sure exactly... back in 1970 a lot of the 'basic scuba' rigs had just the primary reg and (if you splurged) a pressure guage. I remember one of the instructors had a second reg off his primary... the term "octo" was used then... short for "octopus" (i.e, an "octopus rig")... refering to all the "arms" coming out from the central body... (seemed to apply to any rig that had more than two hoses coming off the first stage body...)
... at least... that's how I heard it.
Now, how the alt. reg. started getting called an "octo" is beyond me... there was about a 30 year surface interval in my life between then and a few years ago...
And you are absolutely correct