Incredible. Shudder to think where all these masters went off to. Hopefully not to Universal.
Seems to me that given it was mostly CBS artists, and SQ-encoded, they were probably mixed at one of the CBS studios in New York. So it's possible there's copies of these tapes in the Sony vaults, or of the radio stations like WBCN (Boston) or WCBS (NYC) that originally aired these broadcasts, but I think it's kind of a 50/50 proposition on whether they'd keep this stuff 40 or 50 years after the fact.