Hello Rubiera
If you enter their names into google you will come across some interesting sites, though with one exception they are all in German, the exception that I discovered was the botanical artist, whose works today command quite high prices.
All of the sites will allow you to cut and paste, meaning that you can feed them into one of the web based translation sites.
I myself collect the series, and a number of the female members here do as well.
If you can obtain those that were produced for Berlin I would suggest you do so.
Information about the stamps themselves is spread far and wide, nothing has as yet been collated.
Other than a few specialists the postal history of this issue is very much unexplored.
kind regards
Nigel
I notice that these postmarks have Polish names for the German cities. Why was there this practice in East Germany?
The names are not Polish but Sorbic. The Sorbs are a little slavic minority, who live in the German Lausitz in Saxonia and Brandenburg.
Ein durchaus nicht häufiger Beleg (Ich zeige nur den Ausschnitt).
Original von abrixas
Ein durchaus nicht häufiger Beleg (Ich zeige nur den Ausschnitt).
Mir fällt auf, daß meine Schumann wesentlich weniger Haare trägt!
Maybe Nigel (doktorstamp) can translate the certificate! 
to rubiera:
Her is the translation of the certificate (with some help of Nigel, thanks!)
Definitive", "Clara Schumann", 80Pfg, Michael No. 1305 used. The inscription on the right is brown instead of green, part of the head printing is omitted. This variety is the result of a sideways shift of the print cylinder. Stamp and cancellation are genuine.
