Between 1985 and 1987, Frank Zappa released a pair of mail-order vinyl box sets called Old Masters that contained remastered and restored versions of all of his albums between Freak Out! and Zoot Allures. Each box contained an album of previously unreleased material called a "mystery disc." Although he eventually reworked and remixed a handful of these cuts for subsequent releases, the majority of these recordings were never again released in any form. As part of their exhaustive reissue of the entire Zappa catalog, Rykodisc tackled the mystery discs, releasing 35 of the 37 tracks (the remaining two, "Why Don'tcha Do Me Right?" and "Big Leg Emma," are included as bonus cuts on the Absolutely Free CD) as the single-disc collection Mystery Disc in the fall of 1998. Only two of these cuts were released in identical forms on another album -- eight other songs were available in different edits or mixes, while the other 25 tracks were never released on disc. For diehard fans -- and there really aren't any other kind -- the Mystery Disc material is essential listening, since it chronicles the original Mothers of Invention in a variety of live performances, outtakes, alternate takes, rehearsals and radio shows, all recorded between 1962 and 1969, with the exception of one cut from 1972. It's material that will only be of interest to serious listeners, but there are enough revelations to make the Mystery Disc worth careful listening.
AllMusic Review by Stephen Thomas Erlewine