NOTE: There are two slightly different Hardy Boys books by this title. One was published in the early 1950's, and the other was written in the 1970's. This is one of about 6 instances.
Although some - Including this one - were merely changed (Or 'updated'), some were completely scrapped and the title re-used. The early versions have never been re-printed and are quite scarce, today.
Click on any of the links above to see more books like this one.