Stamp collectors run into many fakes of old issues. Experts estimate that some 90 per cent of Japanese stamps in collectors hands purporting to have been issued before 1880 are forgeries, some are excellent. Collectors buy them knowingly to fill spaces in their albums or to compare with the real stamps and others collect and study the various fakes for their own interest. A cd-rom available from a society of Japanese stamp collectors that details and illustrates these Japanese forgeries prints out to some 3200 pages.
Some of the genuine stamps are worth many thousands.

I understand that it is against U. S. law to deal in or knowingly possess these forgeries, but not certain of the truth of this.