3 different issues of overprinted German stamps.  The stamps of the first overprint series, like the green 5 pfenig on the left, were issued in 5, 20, 50, 75 and 80 pfenig and a 1 mark (similar to the stamp on the right).  The stamps of the second series, shown in the center, were surcharged with an additional value.  The series included a 1 mark on 2 pfenig, a 2 mark on 2 1/2 pfenig, a 3 mark on 3 pfenig and a 5 mark (shown) on 7 1/2 pfenig.  The stamp on the right is part of an issue of 4.  In this series a 1 mark (shown), 1.25 mark, 1.50 mark and 2.50 mark.

As with any valuable stamp, there is a risk that the overprint is a forgery.  The expertizer's mark of Mikulski shown here is commonly encountered.  Stamps sometimes have the mark of an expert on the reverse, seeming to confirm the stamp's authenticity.  Like the overprint, this mark can be forged as well.

After the plebiscite in 1920, Marienwerder became part of Germany and therefor used German postage.  After the re-drawing of the borders in 1945, the area became part of Poland.