Prussian Stamp, Essay, Reprint, Forgerie?

I don't have enough information to identify this stamp.
Its dimensions are 18.2 x 21.5 mm.
White wove paper, thickness 0.09mm with watermark,no gumm.
Thanks for any additional information.

If the stamp is engraved then it is not a forgery.
Michel 3 is printed on dark blue paper so it is not a 3.
Michel 3ND I is also printed on dark blue paper without watermark so it is not a 3ND I.
Michel 3ND II is printed on a greenish-gray paper with watermark. And the stamp is somewhat narrower than the original. So it is likely that your stamp is Michel 3ND II, the 1873 reprint. The paper color may have faded over the last 148 years. But it may also just be the scan is not showing the true paper color.

Thank you Germania for the correct analysis.
Most clues indicate Michel 3NDII (stamp width 18.2 instead of 18.5, watermarked paper, engraved stamp)
The paper is thicker than Michel 3 (0.09 compared to 0.07 mm) and to appreciate the color I scanned it with Michel 12 (white paper)
The color of the paper (colors ... my big problem) remains, I think, the only clue that does not match.

A direct comparison of your first image with the second shows a clear color difference. There is no question that the stamp is on greenish gray paper. It's all in how the scan is done.

The first time I took the picture in the sunlight.
The light was not so strong when I posed for the two stamps together (one with white paper) - in this picture you can see whether or not there is a difference between the colors of the papers.
When I try to identify the color of a stamp, I always do it by placing next to it (and at the same time) a stamp with a known color or a color from the Color Catalog.