What do ya mean "Old Timers?" An 8-track has four separate tracks (programs) of music on it. You press the track button to move it from one track to the next to locate the song you want (in your case, something from Shawn Cassidy or Leif Garrett, I bet). The machine will also automatically change tracks when it reaches the end of one. The drawback to this advanced technology was that sometimes the track ends in the middle of a song and you have to wait for it to click to the next track to hear the rest of the song. MPX stands for multiplex, a prehistoric stereo configuration.
Now where's my bottle of Vitameatavegamin?
https://www.youtube.com/watch?v=xBcdRDB14e0