Dave, this thread has got me searching and learning (sorry for the tangent Brian)...I found this on gearslutz:
Back in the 1970's the Mahavisnu era me and Rex Bogue built a few cool goodies for him
including circuits installed in the Gibson Doubleneck, the custom Double Penis Rex Bogue doubleneck guitar,
a custom 6 channel guitar synth using 6 Oberheim expander modules, effects, etc.
Not sure if thats the synth terry referred to,
I'm still a noob when it comes to John M.'s works.
...on the hunt,
elwood