Yowza! Thanks, all.
Fun update: We know that the seat has the '78/9 stepped profile and not the '75/6 breadloaf one.
I wasn't exactly itching to pick up the period-correct original seat for this bike. Then I saw an opportunity for the right seat in a barn find of pretty-much abandoned early GLs and parts. That barn stash is four hours from my home, a little beyond my standard range for this sort of mission. BUT...
I already had a work-related road trip to that state, and the drive would literally - in the actual and not Gen-Z meaning of that word - take me through the town where the stash is located. I got my work colleague to agree to the pit stop, a 5-minute detour off the highway. (I promised not to get too lost in the hunt to reduce possible friction.)
In the back of the garage, up high on a shelf was a dirty, dusty, cobweb-coated '75/6 seat in a stack of king/queen Corbins/Gentrys and a stepped '78/9 or two. The seat was piled amid the parts that document a story of someone's abandoned plans to put several of these bikes back on the road. It was sad, actually.
Preservation by dust was years in the making. I pulled the seat down. Cobwebs dangled off it like a beard. (I should have snapped a photo because the cobwebbing was October-appropriate and gave the garage the air of a haunted house.
Outside, I quickly wiped the seat cover with a pile of fall leaves to clear the webs and get a closer look. No rips, nicks. Good. It was way too crusty to tell much more than that, and the HONDA logo looked like it had faded away long ago. I bought the seat and a few other hard-to-source items from the piles on the shelf, tossed it all in the back of the car, and continued the work trip.
Once home, I gave the seat a 5-minute cleaning and uncovered a very nice example of a first-gen seat, and the HONDA logo is still quite prominent.
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Whoever got that feverish itch to rebuild or restore an early GL did a pretty good job sourcing components.
I'm not sure I'm entirely done with that garage. But it's a half-day drive from my house.