In meantime, my 1915 Forbin catalogue book does not show this stamp at all. What gives with that!?!?!
The first few sets of Austrian revenue stamps have different designs for each value.
For the 1850 set, Forbin says on page 187 (1915 edition), "Dessin varié pour chaque valeur." i.e. the design is different for each value.
This stamp is from a later issue (without the letters C.M.) but the same is true for these issues too.