This is a German stamp from 1921:

The watermark does not appear to be the regular lozenges (Rauten):

Instead, we see a kind of rosette. The one you can see most clearly is on the left side of the stamp, in the middle. I've processed an image of this area in order to emphasise the distinctive contours of this 'rosette':

Does anyone know what watermark this is?
All assistance greatly appreciated!