As a Carson City resident of 40 years, I have hesitated to offer a  comment 
of the "bar tab" version of the monicer "Mark Twain." However, having  been 
in attendance at such establishments, imbibing moderately of course, I  
think it is, to be polite, "not plausible." In a busy, noisy bar, a "regular"  
would not shout at the bartender, he would make a "V for victory" type two  
finger wave at the barkeep. Possibly then pointing at the glasses to remind 
the  barkeep of what he is drinking, or to insure he gets drinks, not cigars 
or some  other item he regularly orders. And in a quiet bar, would he 
really say, sotto  voce: "Mark Twain, John" (or Joe or Mike or. . .) Seems to me 
not.
 On the other hand, reversing the roles: the barkeep initiates the  issue, 
asking if they wanted refills, which does lead to the bartender asking  
"Mark twain?" [with "more of the same" implied.) California newspapers of the  
day, searchable online, are rife with the use of "twain" as "two". "larger of 
 the twain" "after twain hours' notice" "alleged that the twain are owners 
of..."  Those papers are also full of pseudonyms, the requirement for 
publication  being that the editor knows the real name of the letter-writer.
 
Sadly, Nevada newspapers surviving from that early day in our  Carson City 
history are few. There are, however, references to reading "old  magazines" 
on the trail; Kevin's search for someone having old Vanity Fair  issues is 
not unreasonable. Carl Sagan was the first speaker I heard  admonish me (on 
"Cosmos") that "absence of evidence is not evidence of absence." 
Bob Stewart