Hello JB,
Stereo guitars are quite rare. Stereo wiring was introduced as an option on Guild electric guitars during the mid-'60s, but from what I've gathered so far I believe that the early seventies was the most popular period for guitars with that particular feature.
I think I have less than a dozen stereo guitars from that period in my database, which obviously is only the number of guitars that I've come across so there will be more. However if you really wanted a stereo guitar from that period I believe you'd have a hard time finding one.
Sincerely,
Hans Moust
www.guitarsgalore.nl