I took this from an Auction lot, I did not win. This may be helpful for those wanting early postage rates? I cannot read it of course, but hopefully there is something of value. if it is indeed readable.
These are most common rates used from 01.01.1875 to 31.03.1900. It includes the 50g-100g(Grams) range "Drucksachen" (Printed matter) at 5 Pfennig that started on 01.06.1890. On 01.04.1900, the "briefe" (Letter) rate changed to 20g instead of 15g at the same 10 Pfennig rate.
My best shot: It is a revenue stamp to be used on the card of your medical insurer. AOK Dillingen still exist as a medical insurer. Under Gebuhrenfrei one can still read: Krankenscheingebuhr(insurance card revenue) The gebuhren frei and the overprint of both the 25 indicates that no revenue had to be payed anymore. I have no clue to the reason. Maybe others a bit better informed in German language and revenues of this era can fill in the gaps. Kind regards, Johan.