When did Samuel Clemens become Mark Twain?

Clemens and Twain

Henry Sweets says Samuel Clemens officially became Mark Twain on either Feb. 2 or Feb. 3 of 1863.  That’s the first time an article by Clemens was published with the famous pen name.
Sweets, who is the curator for the Mark Twain Boyhood Home and Museum in Hannibal, MO, says pinpointing the exact day has grown more difficult over the years.

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Sweets also says Mark Twain "was his most famous pen name, it was not the first used by Samuel Clemens."  

Anyone out there who can tell us another one or two?