Hello Dean,
it is just the fact, that a postcard in the 3rd Reich had to be prepayed with 6 Pfennig and not with 1 Pfennig.
So the 1 Pfennig black Hindenburg stamp is wrong.
The second fact is that the postal office would stamp the postcard with a certain stamp of the local post-office - not with a unit stamp.
Feldpost postcards didn't need to be prepayed, however some are prepayed. Mail from the soldiers to their homes was free. In these cases there were no stamps, but the unit-stamp with the fieldpost number had to be on it.
But the card that Phatt shows here, isn't a Feldpost card anyway.
A few examples to show how regular postcards that really had been mailed and delivered by the Reichspost should look alike.
Hope that explain my concerns.
As said before, there is only the chance that this postcard was prepared by somebody who didn't care about post regulations and the card was delivered personally and not by the Reichspost.
Kind regards
Robert