I don't know, Ray. It just seems highly probable that Lewis became aware of one or more of the American sf pulps by the mid-1930s and, having begun to read some issues, kept on doing so for the rest of his life. (He contributed to several issues of
The Magazine of Fantasy and Science Fiction, by the way.)
I've wondered if he sometimes passed on issues to Tolkien or others. I don't think he kept the issues. Thus, as I recall, in the Aldiss-Amis conversation he mentions a story that impressed him but whose author and title he can't remember. It's pretty certainly one of Zenna Henderson's stories from
F&SF. In an article on sf he mentions a little-known story from
F &SF by author and title, so perhaps he saved that issue, or tearsheets anyway -- "Cast the First Shadow."
http://www.tomfolio.com/bookdetailsmem.asp?book=PI05&mem=46
He was really well-read in sf and fantasy, although I suppose a lot of that reading came after he had written his sf novels. His example, of an Oxford and then Cambridge don who was public about his enjoyment of such literature, must have been encouraging for British writers who wrote relatively early books about sf (Amis's
New Maps of Hell, Aldiss's B
illion-Year Spree, etc.).
I'm ranging away a bit from Aldiss, but by the way it was interesting to compare the list of books in the Ballantine Fantasy series that Lin Carter edited 1969-1974 with the catalogue of Lewis's library prepared after his death, and to see how much overlap there was!
Here's that catalogue -- keep in mind it was made several years after his death and doesn't list everything he'd ever owned! But look at some of those holdings for classic sf and fantasy.
http://www.wheaton.edu/~/media/file...s/non-archive-listings/lewis_public_shelf.pdf
This might be of interest:
http://apilgriminnarnia.com/2015/06/17/1950ssf/
By the way -- avoid at all costs the book of Lewis-Clarke letters (called something like From Narnia to a Space Odyssey). It is perhaps the worst-edited book I have ever seen, in that again and again the Lewis letters are transcribed so as to make no sense. If you want to read Lewis's letters to Clarke, they are available in the appropriate volume(s) of the Collected Letters (a 3-volume set). In those books too you will find plums such as Lewis's fan letters to Mervyn Peake, his arch correspondence with E. R.Eddison, etc.