I suppose we'll never uncover all the jokes that carried special meaning
for Twain's family and friends, but I can think of two that surely refer
to his own hair color:

--HUCKLEBERRY FINN, ch. XXV.  "When we got to the house, the street in
front of it was packed, and the three girls was standing in the door.
Mary Jane was red-headed, but that don't make no difference, she was
most awful beautiful . . ."

--A CONNECTICUT YANKEE, ch. XVIII.  Hank has just described a complex
and horrible punishment dished out by Morgan le Fay to a prisoner in her
dungeons:  "Morgan le Fay hated him with her whole heart, and she never
would have softened toward him.  And yet his crime was committed more in
thoughtlessness than deliberate depravity.  He had said she had red
hair.  Well, she had; but that was no way to speak of it.  When
red-headed people are above a certain social grade, their hair is
auburn."

Can anyone think of other examples?

I'm a new kid on the forum.  I'm happy to have joined, and am having a
great time reading through the forum archive.

--Mark Coburn
Durango, Colorado