Claim: The Secret of the Caves,  7th volume in the original The Hardy Boys Mystery Stories, is written by Franklin W. Dixon and published by Grosset & Dunlap in May 1, 1929.
Evidence: The Secret of the Caves is Volume 7 in the original The Hardy Boys Mystery Stories published by Grosset & Dunlap. Franklin W. Dixon May 1, 1929 (original edition)
1965 (revised edition)