A set with great art from the city of Oldenburg in Holstein, not to be confused with the larger city by the name of Oldenburg.
Robert Ball Nachf. in Berlin (the Nachf. stands for Nachfolger which means 'successor(s)') is one of the big numismatic mail-order and auction houses of the 1920s, and they are one of just a few Notgeld dealers from the Notgeld era who actually printed their own envelopes with information about the set contained inside. Some are handwritten, others are typed or mimeographed, and some are even specially printed for the particular note or set that's inside. The envelopes have become quite rare, especially in excellent condition, much rarer than many of the original envelopes that accompanied the sets - to the point that trying to find any particular Robert Ball envelope is usually a lost cause. The company assigned their own catalog numbers to the notes, printed in the upper right-hand corner of each envelope.