When I first saw this stamp I was certain it was a forgery, but after closer examination it turned out to be a GENUINE Type II that was not issued for postal use [1]. Note, however, that this issue exists with fake cancels, often trying to hide the two main characteristics:

1. The "T" in the heading is broken
   
2. There is a white spot behind the ear of the Prince

Forgery	Genuine

1. "Expert: Billig's Grosses Handbuch der Faelschungen, Lieferung Nr. 30: Serbien", O. Stiedl, F. Billig, 1936