At the close of WW2, 1945, the Allies occupied Japan. It was during this occupied period 1945-52 that the marks, Occupied Japan or Made in Occupied Japan were used. In 1952 the San Francisco Peace Treaty put an end to the occupation.
I'm not sure if having those marks make a piece any more valuable or not. I believe it depends more on the individual piece. There are collectors of Occupied Japan pieces though and collect just because of the mark.
And on another note.........there are reproductions out there. On porcelain pieces of Occupied japan you will find the mark under the glaze. If the mark is on top it might very well be a reproduction.
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best...........Tim