MODIFIED Sequential Circuits Six-Trak with Custom Knob Box Controller

via this auction

"Because the MIDI Controller uses one MIDI Input, I have also modified the Six Trak and added MORE MIDI inputs, that that you can connect both the controller and another device (like to a computer). This modification was done by implementing a MIDI Merger from MIDI SOLUTIONS, and in no way is changing any feature of the original unit. It is only adding functions :)

Another minor modification, is to change the power cable (which is normally a gettho cable hanging out of the unit), for a 3 PIN power jack, like you find on any modern synth."

Update via Grant B in the comments. The controller is an M-Audio Evolution X-Session USB MIDI Control Surface.