"His'n" is a slang contracted version of the words "his one," meaning something that belongs to him. It derives from the dialect of the 19th-century American South. Similar contractions appearing around the same time are "your'n" and "her'n." The contraction would seem to refer to a singular possession, but as its usage became more common, it came to be used in reference to singular or plural possessions.
Usage of the word tends to be by rural populations and appears to transcend racial boundaries.
I found the word used in the 1853 book "Weren't no good times: personal accounts of slavery in Alabama," edited by Randall Williams. And Sojourner Truth is quoted as using the word in 1858 in the book "A scholar's conscience: selected writings of J. Saunders Redding, 1942-1977" by Jay Saunders Redding and Faith Berry.
In more current day usage, former Houston Oilers head coach O.A. "Bum" Phillips was quoted in numerous sources in November 1992 as praising legendary Miami Dolphins head coach Don Shula in this way: "He could take his'n and beat your'n; or take your'n and beat his'n."
It's no surprise you found the word in a work by Mark Twain. He was a master of the American Southern dialect.
Hope this helps. Good luck!
Excuse me for responsing so late, Thank you for your comprehensive explanation.
I did find this excerpt from a work by Mark Twain.