potentialities

Yeah, now that you mention it, I think HarperCollins reads that letter as being far more definitive than the text itself will bear. After all, he goes on to say that "So perhaps it does not matter very much in which order anyone read them", which is a statement that will find very little agreement on either side of the Reading Order Debate! :D

As far as I can tell, since 1974 the British Puffin paperbacks had an internal statement about the "correct reading order" opposite the title page where you get information about other books by an author and/or in a series, although they did not have numbers on their (gorgeous!) covers. (I can find lots of pictures of the covers, but none of the inside pages, so my information is not as accurate as I would like.) Whereas the American equivalent ubiquitous paperbacks from Macmillan were numbered with publication order on the covers, which were frankly nowhere near as pretty, which is why a bunch of American Narnia readers flipped out when HarperCollins brought out the new editions numbered in internal chronological order, as it was an idea that I think had frankly never occurred to many of us! The covers are prettier, though.