GK-2 and GK-2a and GK-3 Pin Outpus
When Roland introduced the GK-3 pickup system, they made a few changes to the 13-pin wiring assembly. Most notably, Roland started to implement pin "9" as a way to determine the position of the "guitar/guitar+Synth/synth" output switch.
Roland used the old pin assigned to the guitar signal for the newly implemented pin 9, took an old ground wire and made that support the guitar signal, and rearranged a few things.
Installers take note: the GK-2A 13-pin connector is not compatible with the Roland GK-3 PC board.