THE COMPLETE ESSAYS OF MARK TWAIN and THE COMPLETE HUMEROUS SKETCHES AND
TALES OF MARK TWAIN (both edited by Charles Neider) give the following
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
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
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.