Charley, through a very careful physical analysis of the audio tape, manages to see that something to do with the reality shard nature of the cassette has changed the way this mundane magnetic tape stores data. There's no way a normal cassette reader or player could analyze the data on here because the atomic structure of the tape itself has been altered by exposure to History B. And with that material change, comes a vast increase in the amount of data that can be stored on the tape. Really vast.
The tape contains the fragmented consciousness of Erich Weiss, more popularly known as Harry Houdini, former SAO agent and world's greatest escapist (and promoter). It is more or less permanently in the possession of Operation URIEL at the Lawrence Livermore Labs.