The printing ink is poor, too thin and runny, and as Joe said a Weimar stamp on a war time brassard is very odd. Also, why would a civil authority authenticate a military arm band. The stamp just doesn't make any sense at all.
It's not a matter of weird stamps. A stamp of a civilian city Police HQ has simply nothing to do with military police. That's it. More often than not "weird" stamps have been added to both phoney and genuine items to fetch a higher price. Again, it's not a matter of sloppy printing, this style of armband just didn't exist. These repros are still available (being sold as such) on the market. MP