Asking prices can be very high for these bisects.(lack of 3 Kreuzer stamps during the Austro-Prussian war)
Auction link:lot 102101330000https://www.deider.de/
That being said ,there is no reason why a bisect mailed from a smaller post office would not be genuine.Large Post offices Like Munich had a large stock of different values.They could of used 3 1 Kreuzer stamps also.In local situations halving a stamp was inadmissable but recognized to a certain limit.
The main thing to know is if the stamp & cancel belong together.
The 6 Kreuzer is a common stamp. So if a forger was to create something like this that would be a logical choice.Rarer types like Mi.Nr's: 1II,4I,5aa,5a,13a would arouse suspicion.
I noticed that the cover is somewhat lighter in tone in the corner where the stamp is.
Also the ink in the upper part of the "L" has been in contact with water or something.
Could the stamp (un-halved ) been removed from the cover,halved, and a forged addition of the cancel added ?
Notice the lighter tone in the upper left corner: