This is a stamp that used a common European design, though several countries printed their own versions (Belgium, France, Germany, Italy, Luxembourg, Netherlands, Saar). It's CD1 (Common Design 1). In 1958 the type of watermarked paper that you show (WM304) was in common use.
Though it may not be noted as the specific watermark for the issue in the German Scott catalog for entries 790 and 791, it seems more to be an omission from Scott (or an expectation that the previously noted stamp with WM304 is "continued" through listings until it changes, though WM304 is noted on other entries in the next column). It doesn't seem to be an error, rather the common issue.

CD1 entry from Scott

WM304
Reference:
Scott Standard Postage Stamp Catalog (2016) Volume 3 G-I, pg. 31A
Scott Standard Postage Stamp Catalog (2016) Volume 3 G-I, pg 114