Hi John,
  You are correct.  The setting for the beginning of " A True Story" is
indeed the Quarry Farm porch. The story was written at the Farm and
published in the Atlantic Monthly in 1874.  The Aunt Rachel of the story was
indeed the cook at the Farm.  Her name was Mary Ann Cord.

Best regards,
Gretchen Sharlow
Elmira College
Center for Mark Twain Studies