This photo is of a Twin Reverb amp that was pulled from the GD Pool and given to Simon Babbs and was subsequently sold on eBay many years ago. This amp shows Alembic's version of tapping a twin, meaning this is in an early version and one that Dan Healy actually worked on, he replaced those plate resistors and stated all the other caps are now soggy garbage and should be updated if I owned that amp. I cannot verify and purely speculating this but this could be Jerry's Blackface WoS Twin given to a Prankster, who knows? That said, IMO, it was most likely wired like this one for the WoS. The Dual Twins or any Twin for that matter take a hiatus with Jerry and the GD in 75 and don't return until the day on the green in 76.
1= Normal Channel X
2= Vibrato Channel Y
3= Ground Shunt location when a mono plug is inserted into the tap jack.
When a MONO plug is plugged into either one of them, 1 or 2, that section is tapped and #3 shorts/shunts the input into the power tube section to ground.