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Actually, according to a post later in that thread, part of that project was announced in an unclassified public proceedings in 1974--less than a decade after it happened.  So I suspect that the rest of that story has also probably appeared in a very obscure place and just was not noticed outside of a very small community.  Unknown is not the same as classified.  Admittedly, the story of the Mogul balloon project was not declassified until the 1990s (associated with Roswell).  But 60 years is an awfully long time for a flight project to remain classified.