This series was issued for use on Prussian official mail beginning 1 April 1920. On 1 July 1920, these issues became valid for use Germany-wide.
The stamps are similar to the later Numeral Issues which were valid throughout Germany, except for deletion of the franking number “21” in the corners.
MiNr. 21 can be found with a plate flaw — broken line under the “M” in “Marke”.