
Update via Kaden in the comments:
"heh? Walter Sears was producing boutique guitar synths in about 1973 (as I recall, Steve Howe from Yes had one with an elbow controller), which led to the 360 Systems guitar synth in (I think) 1976. There was a band called Symphonic Slam that made a *huge* deal out of having one. Sounded kinda Arpish, with appalling pitch tracking."