This is almost right in the middle of Jerry's Mesa Mk I period, which ended 10/3/76 and began either with the recording of Allah 3/75, or possibly with a Legion show earlier that winter.
Kahn's Showman is seen in many, many JGB shots. Usually, you can also see the MC2300 it is feeding. It's odd because this Showman headseems to sport BF colors, but the Fender logo and the knobs are clearly the Blonde or Brown style, pre-CBS stuff. It's probably a 62 or 63, with the original tolex replaced with black... not sure about the grill cloth.
I don't recall seeing an actual Showman cab w/ it before, or the pair of HT 2x12's. I do seem to recall an HT 2x15... These 2x12's might be piano (closed-back) cabinets out of the WOS... or Hard Truckers built specifically for JK.
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia