I had another look through Wirnsberger's directory and your stamp becomes even more puzzling; the crown to my mind is definitely a german crown but the palm fronds are very similar to a stamp used by St Etienne. The only catch is that french crowns are castellated in other words have 4 square posts on the top like the walls of a castle and are very different from german crowns.
The crowns used in stamps of different countries are usually different as opposed to a single type of stylized crown fits all.
cheers Doug
