According to antique collectors, via AntiqueWeek 3/13/95 central edition,
"Nippon" was used on Japanese porcelain to comply with the McKinley Tariff Act
of 1890.  This was accepted by the U.S. trade officials until 1921, when "in a
stricter interpretation of the McKinley Act, it was ruled that Nippon was a
Japanese word; the law required the country of origin be in English."
Appearently, U.S. officials decided that Japan was the English word.  Thus,
Japanese porcelain with the "Nippon" mark dates before 1921 and probably after
1891.  Japanese porcelain marked with "Made in Japan" dates after 1921.  It
should be noted that the referenced article was written in reponse to the
numerous fake "Nippon" porcelains being produced by the 1980s.
 
Hope this helps!
 
M. Jay Stottman