What became of the rings buried in the Magician's Nephew?

Question

Toward the end of the Magician's Nephew, Aslan told Digory and Polly to bury all the magic rings, and they buried it in a circle around the place where Digory planted the core of the apple he brought from Narnia to cure his mom. Quote from the last chapter the book:

"I've got them all," said Polly. "Look, it's all right, I'm wearing gloves. Let's bury them."

"Yes, let's. I've marked the place where I buried the core of the Apple yesterday."

Then Polly came over the wall and they went together to the place. But, as it turned out, Digory need not have marked the place. Something was already coming up. It was not growing so that you could see it grow as the new trees had done in Narnia; but it was already well above ground. They got a trowel and buried all the magic rings in a circle round it.

My question: While the last chapter explained how the Wardrobe came from the tree, did C.S. Lewis explained either in the Narnia books, his letters, interviews, or his other writings what became of the buried rings?

Answer 1

In The Last Battle, King Tirian appears in a vision to the Seven Friends of Narnia, which leads to them trying to get back to Narnia by using the rings. From the chapter How Help Came to the King

JILL: The next question was how to get here. You can't go just by wanting to. So we talked and talked and at last the Professor [Digory Kirke] said the only way would be by the Magic Rings. It was by those Rings that he and Aunt Polly got here long, long ago when they were only kids, years before we younger ones were born. But the rings had been buried in the garden of a house in London and the house had been sold. So the problem was to get at them. You'll never guess what we did in the end! Peter and Edmund -- that's the High King Peter, the one who spoke to you [in the vision] -- went up to London to get into the garden from the back, early in the morning before people were up. They were dressed like workmen so that if someone did see them they'd look as if they'd come to do something about the drains. (snip)

And they must have succeeded for the next day Peter sent us a wire to say he'd got the Rings.

They were supposed to meet at a train station so they could all try and use the rings to get to Narnia. However

all of them are killed in a railway accident before they can try to use them.

It's not clear what became of the rings after that.