Here is piece of Prussian postal stationery which has been surcharged by use of a North German Confederation one groschen rouletted stamp. The area of the surcharge was then typeset overprinted in gray with a field of tiny, "NORD DEUTSCHER POSTBEZIRK" impressions in a 28mm x 26.5mm field.
The postal envelopes were made from remainders of German States postal envelopes after the creation of the North German Confederation. They exist for some envelopes of Braunschweig, Mecklenburg-Strelitz, Oldenburg, Prussia and Saxony. Michel warns that there are many forgeries of these "use up" issues. Just about every part of them has been faked.
The envelope shown may be NGC Mi #24 or U28, depending on the length of the gum on the flap. Michel's Ganzsachen-Katalog Deutschland
catalog gives no help in the listings, but warns that this feature also has been faked.
This item is a great example of how good some forgeries of German material can be, if it is a fake. I paid a few dollars for it some years ago. The NGC stamp appears genuine, the Cochstedt and receiving cancels look good, the gray overprint is very precise and clear, and the Prussian envelope appears to be correct.