"When the music in this boxed set was recorded between 1955 and 1963, Columbia engineers typically ran two tape machines. The approved performances were spliced from tapes recorded on the A-machine while the tapes from the B-machine were left un-spliced — serving as a backup in case anything happen to the A-tapes.
To duplicate the sound of the original mono albums on the nine CDs in this set, all of the A- and B-reels were pulled from Sony's Vaults. Then the best-sounding reels were selected and a series of digital remastering techniques were applied — using pristine copies of the original LPs as audio benchmarks."
Mini- LP Replica Jackets