I forwarded the earlier post regarding the imaginery St. Petersburg
to a colleague who has studied early maps of Missouri.  I thought
the Forum might be interested in his reply which I am pasting in
below with his permission:

~~~~~

In an 1820 atlas by C.V. Lavoisne there is a map of the United
States drawn by John Melish and engraved by Benjamin Tanner which
includes a town in Missouri called Petersburg located south of Fort
Mason and north of St. Charles. Petersburg appears to be on the
Mississippi River but due to the scale of the map, it may have been
some distance west of it. Petersburg may have been an early name for
the present day town of St. Peters, Missouri. Mark Twain may have
known of this early community under that name but whether it influenced
his renaming Hannibal to St. Petersburg is not documented.  (Dave Thomson)

~~~~~

If anyone on the list is familiar with the site or history of the early
Petersburg, MO I would be interested in knowing its fate.

Barb