Many, many thanks to the helpers on this thread. Indeed, the job did go quickly, about 45 minutes to do each seat. Patience is key. C-Clamp method popped those eight bad boys right up a quarter inch, such that the fins were up and out of the gunwhale wood, and could then turn. Nuts a little reluctant to come on and off the bolt ends, but persistence and persuasion helped that happen, too. I never could have done this without all the help above. Thank you.