There isn't anything obvious on the books to indicate that they are for sale in the US. So how do we know? A recent article of mine in Paperback Parade #71 discusses the evidence. There is a good deal of circumstantial evidence but the the key evidence is the distribution codes on the two books. All but the earliest Canadian books have "MDS" on the bottom of the spine. The US books have "KN" or "KNC". These stand for Magazine and Distributors Sales Ltd. of Toronto and Kable News Company of Mount Morris, Illinois, Export's wholesale distributors in Canada and the US respectively.
Twenty of the 28 US books were published with and without a dust jacket. DJs on mass market paperbacks have always been uncommon. A few US publishers had a very rare DJ from time to time but Export's 20 out of 28 makes this series unique in Canada and the US.
Below are a few examples of this series, first without the DJ, then with.