THE COMPLETE ESSAYS OF MARK TWAIN and THE COMPLETE HUMEROUS SKETCHES AND
TALES OF MARK TWAIN (both edited by Charles Neider) give the following
dates:
  The First Writing Machines (1905)
  Italian With Grammer (1904)
  A Monument to Adam (1906)
  Introduction to 'The New Guide of the Conversation in Portuguese and
English (1883)
  Portrait of King William III (1871)

Bruce McElderry, Jr., CONTRIBUTIONS TO THE GALAXY: 1868-1871 indicates that
"An Entertaining Article" was first published in the December 1870 issue of
THE GALAXY.

I haven't been able to pin down "A Letter to the Secretary of the Treasury"
but the piece itself contains a date of 1902.