The Levin brothers were childhood acquaintances of Twain.  They lived in
Hannibal.  They were nicknamed "Twenty-two."  Henry Nash Smith argues that
they
may have been the basis of the name "Forty-four" in "Schoolhouse Hill" and
"44" in "No. 44, The Mysterious Stranger."  See William M. Gibson's note to
184.33 in the California edition of *The Mysterious Stranger.*