> and the other is--to the best of my knowledge--the old JMT. . .

I agree. That would explain why we saw the old, split sign, on a bent-over tree labeled "Muir Trail" at a trail junction east of Crater Meadow. And farther along, high on a tree, a thin white metal band with "John Muir Trail" on it. These are both NOT on the current JMT.