Thanks for all the thoughts and suggestions.
To answer a couple questions that have been asked:
I got the amp from a friend of a friend about 1979. I think I paid him $100 for the amp and a Woodson 2x15 cab loaded with 2 JBL D140s. Combined, I think the two pieces weighed in around 150 lbs. I was tipping the scale at about 135 lbs back then. The Hiwatt alone weighs about 50 lbs. I gigged with my Rick 4001 and that rig for a couple years, then built myself two 1x15 reflex cabs, modeled roughly after the Sunn reflex cabs that John was using at the time. I gigged that setup for another 7 years or so, then got a smaller 1x15 cab and retired the Hiwatt in favor of a much lighter (but sonically inferior) Hartke head.
The suggestion that this could be a Leeds amp was raised by a previous commenter. Believe me, I consider that a very outside possibility, at best, and have seen some photos of the Leeds show and, while none that I have seen show John's rig clearly, I am doubtful (though it's a pretty nice fantasy). In the shot that was shared of Pete's rig at Wight, the top amp head looks exactly like mine. I have seen another shot of John's rig somewhere that shows an amp that looks like mine, but can't for the life of me remember where I saw it.
This has been a very fun journey of discovery. I've learned a lot more about the amp, all of it good. I can't thank Clayton Callaway at HiTone enough. He has provided a wealth of information and shared his vast knowledge and love of early Hiwatts generously without hesitation.
I really just need to figure out what comes next. I didn't think to get in touch with Steve Luongo (John's drummer in his last solo band), and that's a good idea. I have met and spent time with Steve when John's band was touring, and actually brought up the subject when I last talked with him, before John's death, but it didn't seem to go anywhere. I have also thought of trying to contact the Experience Music Project in Seattle to see if they have any ideas. Much to chew on...
Please keep postings thoughts and suggestions. I'll update as things happen.