What does the word 'dot' -- in quotes in the original -- refer to?

DDD

https://archive.org/details/jstor-25119224 ; or
http://www.online-literature.com/twain/3263/

"Rich American girls do buy titles, but they did not invent that idea; it
had been worn threadbare several hundred centuries before America was
discovered. European girls still exploit it as briskly as ever; and, when a
title is not to be had for the money in hand, they buy the husband without
it. They must put up the "dot," or there is no trade. The commercialization
of brides is substantially universal, except in America. It exists with us,
to some little extent, but in no degree approaching a custom."