Funny you should ask about that.  Just had my class to the rare book room
here to show them some first editions and talk about subscription
publishing.  Our conservator did manage to get a hold of 4 "salesman's
dummies" from the period.  Though none of the latter were dummies of
Twain's own books they are fascinating things and apparently still
circulate.  One had a bizarre composite binding to display the various
options to the customer (e.g., leather, cloth bound, etc.).  Each had a
short excerpt of the prospective text.  Then, at the end of the dummy, one
finds ten or twenty pages of blank ledger sheets upon which customers would
literally subscribe; at the top of each ledger, as an added enticement,
there appears the names of a half a dozen famous persons who had already
signed up for a copy.

Perhaps the Twain Papers in Berkeley holds "salesman's dummies" for Twain's
own books?

Brian Collins
Haverford College