Into the Wardrobe

No, this is not another thread about Susan's fate. I'm just trying to find the quote where Lewis said "she might make it in on her own right." Where and when did he say that?

Thanks!

This can be found in a letter to Martin Kilmer, dated January 22, 1957 - p. 826 of vol. 3 Collected Letters or p. 67 Letters to Children