^ Also, some of the Narnia books had special revisions for the American versions made by Lewis during proofs. These have since been “standardized” so that, though modern printings of the American Narnia books still have American spellings, otherwise they have all the same text otherwise as the UK English versions.
But, books and movies being what they are, paperbacks of the original US Narnia editions are still far easier to come by than high-quality copies of the OOT.