Multiple printings are required for a note. Typically, the backs are printed first, then the front, then the seal and serial numbers. For this reverse changeover to occur, a 1934A sheet was over-printed immediately before a 1934 sheet -- with the series going "backwards" or "reverse" from 1934A to 1934. A mule is created when the back of the note is from a different series.
Not only is this pair a reverse changeover but one of the notes is a mule - a very rare occurence!