cool photo, thanks for sharing. looks like Jerry might be playing the '57 sunburst strat that he used onstage during summer of '72...presumably while Alligator received further modification. more like that one please!
here, it looks like the two twins are stacked in this fashion in order to make a single column of four 12 inch JBLs, and I guess this is also right around the time they began the practice of stacking all the speakers in their sound system in vertical line arrays, working their way towards the Wall. 8/27/72 he still has the pyramid stack of 3 Twins, then the 10/21/72 pics is where I first see the 3 Twins stacked end to end vertically. soon after, he had the Alembic cabs. the two-stack in the pic, he might have used for his club gigs and rehearsals around mid-late '72 is my guess... it's actually very rare for this particular time period to get this clear a view of Jerry's amps!
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia