SB24
Tonight, I neutered my SB24.
Somehow, in the process, I apparently repaired the malfunction. My only guess is that there were small gaps between critical pieces of metal somewhere in the foot that had been compromised by handling, age, or even heat.
Taking the foot off was a matter of seven screws total. I removed the TTL pins, taped them up and stowed them inside the plug compartment, and reassembled. It took two tries, because the first time, I failed to properly tighten the three internal screws.
There’s a nice little tutorial over on Sean’s site for removing the unnecessary TTL pins. Maybe it helped with my problem, but I bet it was the dis-assembly and reassembly process that made it happen.