Our collective best guess when we published Letters 1 was that Clemens was
off by a part of year in saying he raised the pistol in 1866. The piece of
contemporary evidence that might correct the year is his letter to Orion
and Mollie written in October 1865, especially the last page, in pencil,
written on 20 October 1865, and in that, especially the last paragraph:
You are in trouble, & in debt--so am I. I am utterly miserable--so are you.
Perhaps your religion will sustain you, will feed you--I place no
dependence in mine. Our religions are alike, though, in one
respect--neither can make a man happy when he is out of luck. If I do not
get out of debt in 3 months,--pistols or poison for one--exit me. [There's
a text for a sermon on Self-Murder--Proceed.] (Letters 1, 324)

Bob Hirst