Dan Healy is selling it, the guy who modded most of Jerry's rig. It does not have the 90's rack mod because it was not the one used in his live rig, at least post aug 79.
Closing of Winterland new yeara eve 78 shows the gray knob in use as well.
" The more elements you have stable, the more you can concentrate on your playing and not be continually adapting your technique to your equipment." ~ Jerry Garcia