From Daniel Morley McKeithan’s book, “Traveling with the Innocents Abroad, Mark Twain’s Original Reports from Europe and the Holy Land”, 1958, I’ve abstracted the letters and provided pdf clips from the archived newspapers. My copy of McKeithan’s book is a used copy found on Amazon. From a signature on the front cover page I can see that it was a Christmas gift (1958) from Frances. I am unable to discern the recipients name. The Innocents Abroad was published in 1869. It was, apparently, based on these letters sent to Daily Alta California, the New York Tribune, and the New York Herald, although I recall reading some where that Twain claimed he did not need the letters to write the book. From McKeithan’s introduction:

Of the fifty-eight letters which he [Twain] wrote concerning the trip, all of which are included in the present volume, fifty were written for the San Francisco Daily Alta California. These were numbered by Twain as follows: 1-39, 43-48, and 51-54 (the last letter, written after the return of the Quaker City to New York, was not numbered). Numbers 40-42, 49 and 50 are missing, possibly because Twain got his numbering confused – he was constantly moving, the weather was almost unbearably hot in the Holy Land, and he was sometimes irritated by guides, hostile natives, swarms of beggars, and even by the pious pilgrims themselves. Of course it is also possible that a few letters were lost or that the Alta editors decided not to publish several of them. The amazing thing is that he was able to write so many letters and so well – including the best writing he had done so far. Letter 13 was written for the Naples Observer but was reprinted by the Alta. Letter 58 was written for the New York Herald upon the return of the Quaker City. The other six letters (2, 12, 23,26, 38, and 56) were written for the New York Tribune.”

We need to be careful about the numbers mentioned by McKeithan.  Sometimes he is referring to his chapter numbers and sometimes to Twain's numbering of the Daily Alta Letters.  This caused me a bit of confusion regarding what McKeithan refers to as the thirteenth letter, the Quarantine Letter.  It is not a letter Twain wrote to the Daily Alta but to the Naples Observer.  The Daily Alta reprinted the letter.  McKeithan obtained photostats of all the letters from the Library of Congress, I found mine on on-line archiving services: The Daily Alta California: California Digital Newspaper Collection, Center for Bibliographic Studies and Research, University of California, Riverside, . The New York Tribune and the New York Herald: Newspapers.com by Ancestry: https://www.newspapers.com/