... and so is the case with this item, an agent's certificate from the National Ink Co. issued December 21, 1865. The philatelic usage is mundane, although it's nice to have a clear "N. Ink Co." manuscript cancel.
No in this case, it's the certificate itself that has all the appeal. A patriotic eagle vignette, and more importantly 3-color printing in red, blue, and metallic gold. That level of expense was by far the exception rather than the rule in that era.
Gorgeous piece IMO. Click on the image below to view it at higher resolution.