At left is a close-up view of a large die proof of the 15c 1866 issue, generally referred to as the first U.S. mourning stamp. This stamp was issued well over a year after Lincoln's assassination. 

Below is a close-up of the National Bank Note Company imprint which appears below the die print. 

A reduced image of the entire proof is pictured at bottom. The dark line below the imprint is actually a shadow of the die sinkage upon the india paper, a byproduct of the scanning process.