This is an old thread that I meant to respond to earlier.  I
use some of the ceramic books published by Collector Books.
They are all aimed at collectors, of course, but they have
some useful information in them.
 
The Collector's Encyclopedia of Nippon Porcelain by Joan Van
Patten, Collector Books, 1979.
 
pp. 26-31, Marking systems states, talking about the McKinley
Tariff act of 3/1/1891: the word Nippon was backstamped on
most items exported from Japan to this country until 1921.
However, we don't usually find the word Nippon all by itself.
It is generally used in conjunction with many different
markings, some symbols (which both .. Japanese and English
speaking people could both understand), and some writing.
Deciphering these marks can be quite difficult at times. ***
[I]n addition to the word Nippon the backstamp may contain the
name of the kiln, the potter, a symbol representing a family
name, names of towns, sections or provinces of Japan, marks of
agents, family concerns manufacturing the wares, or exporter
or importer marks."
 
Also, apparently many porcelain items were imported as white
"blanks" and decorated in the United States.  Again, van
Patten:  "Items stamped with just the word Nippon or
Nippon/Noritake on them generally were 'in the white' blanks
decorated elsewhere. ... Backstamps with letters and initials
seem to be those of the importers in the United States and
Canada.  There were many companies in the United States
importing these wares during the years of 1891-1921.  Besides
the Pickard Co., these were Louis Wolf and Co., NYC & Boston,
the Spicer Studios in Akron, Ohio, the Jonroth Studios, and
Yamato Importing Co. in Chicago to mention a few.  The
Noritake Co. exported many wares marked with their name and
the Okura Art China Co. ... with their OAC mark."
 
Apparently, the 1921 law had decided that "Nippon" was a
Japanese word, and would not be allowed into the United States
with that mark after 9/1/1921.  Obviously, some Japanese
manufacturers must have seen the writing on the wall earlier
and began putting "Made in Japan" before the official date.
 
Hope this information is still of interest.