I've found that the series three books were published in separate editions in Canada. They look very similar to the U.S editions.

Here is the back cover of the U.S edition of "Gateway to Doom".

Now here is the back cover of the Canadian edition of 'Gateway to Doom". It's really hard to tell in this picture, but if you look on the right side you'll notice itsy-bitsy letters. These say "Printed in Canada". The funny thing is, if you open up either the Canadian or U.S. editions and look at the copyright page, you'll see that they both say manufactured in the United States. So which is it? Was it manufactured in the United States or was it printed in Canada? :-)