When the King and Duke come on board the raft, Huck tells them that he owns Jim, but has to travel by nights because "people was always coming out in skiffs and trying to take Jim away from me, saying they believed he was a runaway." While the King cons the camp-meeting at Pokeville, the Duke prints up this counterfeit wanted poster, identifying Jim as having run away from a plantation below New Orleans. The Duke says it will give them a cover story for their travels, allowing them to pretend they're taking Jim south for the reward; later the pair use it to con the Phelpses into buying Jim for $40 with the intention of collecting the reward.
MT's older contemporary readers would have recognized how Kemble derived the Duke's poster from the standard way by which fugitive slaves were advertised in ante bellum newspapers and wanted posters.
The Barrett Collection, UVA PS1305 .A1 1885b