I think the session masters were there, which is why unissued material was lost. Copies of the album masters, which are the tapes mixed down for LP from the session tapes (with lovely RIAA curvature and all) obviously did survive in other copies, which is why the albums can be reissued from them, albeit without any extra tracks of unissued material, as the unissued stuff existed only in the studio master. Mind you, I did just have a quick look inside a couple of Atlantic Mosaics and didn't see any discussion of this issue.