Mere Christianity Chapter 7 ("Let's Pretend") Book IV: Reference to Story of Mask In Mere Christianity Chapter 7 (&quot;Let's Pretend&quot;) Book IV, Lewis writes that there is a story

about someone who had to wear a mask; a mask which made him look much
nicer than he really was. He had to wear it for year. and when he took
it off he found his own face had grown to fit it. He was now really
beautiful.

Does anyone know what this story is?
Thanks
C.S. Lewis won a triple first (the highest honours in three areas of study – Classics, Philosophy and English) at Oxford. He then taught as a fellow of Magdalen College for nearly 30 years. Later, he was the first Professor of Medieval and Renaissance English at Cambridge University. Much of his academic work concentrated on the later Middle Ages, especially its use of allegory. His book “The Allegory of Love” (1963) helped reinvigorate the serious study of late medieval narratives. His last academic work was “The Discarded Image, An introduction to Medieval and Renaissance Literature” (1964). In his book “Fern-seed and Elephants” Lewis picks a statement from a liberal commentary where John’s Gospel is called a ‘spiritual romance’, ‘a poem not history’. He retorts: “I have been reading poems, romances, vision literature, legends, myths all my life. I know what they are like. I know that not one of them is like this.”
I would suppose that this second story about a person having to wear a mask for years, comes from that wealth of late Medieval literature with which he was so conversant. It is hardly necessary to track it down; he knew of that story and incorporated it into chapter 7 of his book as an illustration of what he was going to explain:

“What had begun as disguise had become a reality. I think both these
stories may (in a fanciful way, or course) help to illustrate what I
have to say in this chapter.”

That story might have been, basically, just what he said it was, in his first paragraph of chapter 7. If the original tale had been more elaborate, Lewis would have expertly condensed it to its salient points. He was just using it (and the summary of Beauty and the Beast) as a spring-board from which to launch into application of knowing theology. If you could find the original story, it would not have anything further in it that would improve its application in Lewis’s chapter. I’m not so sure that his application is sound, but I have no doubt that his condensed version of that un-cited story is a neat summary which helps in his explanation of applying theology in order to become more Christ-like.
His conclusion is important, for he says there has been something very misleading in his language up till that point.

“I have been talking as if it were we who did everything. In reality,
of course, it is God who does everything. We, at most, allow it to be
done to us. In a sense you might even say it is God who does the
pretending. The Three-Personal God, so to speak, sees before Him in
fact a self-centred, greedy, grumbling, rebellious human animal” and
acts as “the higher thing” which “always raises the lower. A mother
teaches her baby to talk by talking to it as if it understood long
before it really does.”

That ties the opening illustration (of wearing a mask for years) with the conclusion, that God can transform a horrible person into a Christ-like one.