Although the 24c denomination was introduced in 1857, it was not put into use until 1860. It strongly resembles the design of the 1857 5c. As in the 5c the top and bottom parts, plus portions of the side design, have been cut away to fit the design into the plate.
It’s primary use was for mail to England, the rate being 24c, previously two 12c stamps had been commonly used. It was also frequently used on the Prussian closed rate postage.
The Swedish Tiger’s US Stamp Site An illustrated Guide
Siegel Encyclopedia (1857-60 Issue)
Civil War Timeline, at Siegel Encyclopedia