Now that I've had time to think about it, I'm almost sure of what happened.
The only curious part of the roll is the very inside, the last 6 inches or so, were not exposed. Everything else was solid black. There's one good way to accomplish this: open the back with the camera in the middle of the film roll.
If I had just overdeveloped it, there would be black frames but there would still be white borders, the space between the frames that was never exposed. This wasn't like that, it was all black.
So someone at some point opened up the back of this old camera and the film was still inside, most of it already taken. There might have been one frame left. In fact I would have done this myself if the film were still mounted (although I would have closed the camera and salvaged the rest of the film except maybe three frames). But maybe someone who didn't know about film did this and then proceeded to remove the film from the camera and wind it up on the takeup roll. This would expose everything except the last few inches of the roll, which were never unrolled.