Apparently, MCA - which is really the first of a mega-conglomerate labels as we know them today - was concerned about shelf space, and they supposedly trashed multi-track masters and mono mixes from all of the labels the acquired, but simply kept the mixed stereo masters for future use. I would not be surprised if a lot of the fringe music that I love was entirely trashed - mono, stereo, multi-tracks - before it was ever digitized.