On one side of this postcard MT is "Mark Twain"; on the other, he's "S. L. Clemens."
Below: "Mark Twain" is writing to tell a correspondent that he declines an invitation to lecture, and if you look closely you can see that the whole message is printed rather than written. In other words, at some point (probably in the mid-1870s) MT had a batch of cards printed up to look as if he had written them, with the generic "Hartford, Thursday" at the top and a facsimile of his signature as "Mark Twain" at the bottom. It was in 1872 that he first decided to retire from the lecture platform. Even on this one side, "Clemens" appears in the "SLC" design in the corner.
Bottom: "S L Clemens" is using the blank side of the printed card, where the address would have gone, in August, 1882, to write Charles Erskine Scott Wood (the West Point officer who had 1601 printed clandestinely by cadets) to accept "it" -- which may have been a copy of 1601 for himself, and another for MT's friend Joe Twichell. The darker handwriting at the top is Wood's, who scrapbooked all his communications with MT about 1601.
MT often re-used paper for his correspondence. I don't know how often he sent out mass produced copies of his signature. He never really "quit the platform."