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This must have been part of what had occurred:

Japanese government intervention in the currency market

In the 1970s, Japanese government and business people were very concerned that a rise in the value of the yen would hurt export growth by making Japanese products less competitive and would damage the industrial base. The government therefore continued to intervene heavily in foreign-exchange marketing (buying or selling dollars), even after the 1973 decision to allow the yen to float.

Despite intervention, market pressures caused the yen to continue climbing in value, peaking temporarily at an average of ¥271 per US$1 in 1973 before the impact of the 1973 oil crisis was felt. The increased costs of imported oil caused the yen to depreciate to a range of ¥290 to ¥300 between 1974 and 1976. The re-emergence of trade surpluses drove the yen back up to ¥211 in 1978. This currency strengthening was again reversed by the second oil shock in 1979, with the yen dropping to ¥227 by 1980.

http://en.wikipedia.org/wiki/Yen