HOW THE SEQUEL DETERMINED THE TIME SETTING OF HUCK FINN

Why did Mark Twain change the setting of Huck Finn by a full decade after he wrote it?  The answer can be found in one book from his personal library. 

After submitting the original manuscript of Huckleberry Finn to his publisher, Charles Webster, he wrote to him on July 6^th, 1884, "Send to me, right away, a book by Lieut. Col. Dodge, USA, called ’25 years on the Frontier’ --- or some such title --- I don’t remember just what....  Send what you can find.  I mean to take Huck Finn out there." 

Eleven days later he had Our Wild Indians by Lieut. Col. Dodge, USA, in his hands, and wrote Webster, "The book you sent is the right one... Don't need any more Injun books."  Mark Twain immediately began brainstorming Huck’s Indian adventures as he read it, & wrote notes on its pages as ideas sprang into his mind.  By page 68 he determined the time setting of Huck’s sequel was going to be 50 years ago (from 1884) & wrote “our time” along the left blank margin.

  
A week later he recalled that Adventures of Huckleberry Finn was set precisely 40 years ago (in the mid 1840’s).  So on July 24^th he ordered Webster to “… alter the title-page so as to say, ‘Time, forty to fifty years ago’ instead of ‘Time, forty years ago.’ ---- If the printing isn’t begun, you can make the alteration, of course – so do it.”  The change was made for the explicit purpose of having the settings of both stories match for the sake of historical realism.