I cannot help with the Gibbons numbers. But the 50 pfenning was issued with two different watermarks(in German Rauten & Waffeln). maybe somebody else can help with a accurate description of the watermarks in Engelish and the numbers. Kind regards, Johan.
The 50 pfennig stamp is either Scott 143 (watermarked "lozenges") or Scott 167 (network watermark) and the 75 pfennig, deep ultramarine is Scott 170. They were issued about 1921, just before the hyperinflation that destroyed the German economy. The others with denominations in the millions of marks were issued around the time inflation was really starting to kick into high gear, with the highest denomination being 50 milliard (billion) marks, issued in late 1923. Just think, one stamp which might have just about been enough to mail a letter had a denomination which a few years earlier might have been close to the GNP of the entire country! If you look online you can find pictures of German children playing with bundles of inflation era banknotes used as building blocks. It is interesting to note that it is difficult to find genuine postally used stamps of German inflation stamps with some selling for hundreds and in a few cases, thousands of times as much as the unused stamp. An example Scott lists #267, 500 marks light green overprinted 800,000 marks at 25 cents unused, but $1,500 postally used.