P.P.S. Billboard to the rescue! The April 6th, 1974 issue lists the first six volumes in this series, including this one, on the New LP/Tape Releases pages, all appearing under Popular Collections on page 69. Given that Billboard was post-dated, release in either late March or early April was likely.
P.S. There are no copyright registrations for this or any other Blues Spectrum release in 1973 or 1974 at all; maybe Johnny Otis didn't believe in such niceties!
The 1974 date possibly comes from Discogs - the entry there shows 1974 without a source, and also without label images, but this was also released in France on EMI's Columbia label, and that version shows (P) 1974 on the labels and (P) 1975 on the back cover. The correct year might be 1973, if the UK issue is anything to go by!