Whenever I need to consult the original 1966 edition of Letters of C.S. Lewis - usually to quote from W.H. Lewis's memoir of his brother which prefixes the letters - I look through the letters themselves and note the ones whose addressees are anonymized, usually as "A Lady." And then I remember the 3-volume supposedly complete Collected Letters, which eschews such coyness. This time, while I had the 1966 book out of the library, I decided to collate the anonymized letters against the Collected Letters - which I own; it'd hardly be possible to do my research without it - and find the answers.
So "A Lady" refers to various persons; before 1945 it's usually Mary Neylan; afterwards it's most often Mary Van Deusen, sometimes Genia Goelz; for a stretch in 1952-6 it's usually a Mrs Johnson of whom nothing else is known. "A Godchild" in 1949 is Sarah Neylan, who reappears as "A Child" in 1950; "A Child in America" in 1956-7 is Joan Lancaster. I should look some of these people up.
"A Friend, who was troubled about a younger woman's unsuitable devotion" in 1946 turns out to be Owen Barfield. That I really ought to search out in Barfield's biography.
I'm writing about this now because the 1966 book needs to go back to the library.
Most surprising, however, is the presence in this book of 5 letters which, as far as I can find, are not in the Collected Letters at all. Not all the letters are dated, and sometimes finding them in the Collected Letters requires recourse to that work's index, but it's a very complete index, so when I say that somehow these got missed, I can be pretty sure of it. A couple more letters aren't in their proper chronological place in the Collected Letters but were added to the appendix, which suggest that nobody consistently combed the 1966 book when compiling the Collected Letters.
One of the missing letters, of December 1962, has some interesting material on Lewis's intent while writing the Narnian books, and another, of April 1959, is the letter to Peter Bide (unnamed here) in response to Bide's request for prayers for his sick wife, a letter Lewis refers to having written in several other letters of the time, as I found while looking unsuccessfully for it in the Collected Letters.