To fully answer your question would take days of research at the MTP. Twain
endorsed many products, but unlike modern day "endorsements" most of these
were not paid endorsements. In fact, many of them were not even endorsements
given with his permission. His name, image, and writings were used endlessly
without his permission to promote a variety of products --automobiles,
cigars, whiskey, foods, clothing, pens, vacation resorts, other authors'
books, charities, schools, etc. I have piles of such advertising from his
lifetime and sorting through it all and determining which were done with his
knowledge, without his knowledge, and which might have involved any payment
would be a huge task.

If you restrict yourself just to endorsements made after the MT Co was
organized, then you miss four decades of endorsements made before the MT Co
was formed (in the last couple of years of Twain's life, with Ralph Ashcroft
"granting" the endorsements in Twain's name). I think they got as far as a
MT Cigar and a MT whiskey (but MT cigars were being marketed in the late
1870s...). I'm sure the MTP would have the documentation you seek for that
brief period.

If it's any help, Twain's interest in trademarking his name in the early
1880s was motivated by his wish to prevent reprints of his writings under
his name rather than exploiting its use for endorsements. I have a letter
Twain sent Chatto & Windus on this topic when they were about to publish
Life on the Mississippi. I think the MT trademark might have made its first
appearance on the verso of the title-page of Chatto's edition of LoM, but
I'd have to double-check to be certain.

Kevin Mac Donnell
Austin TX