SCAP (Supreme Commander for the Allied Powers, though it is used for the occupation government as a whole rather than just MacArthur) issued SCAPIN 1535 in February 1947, which directed that all Japanese exports were required to be labeled "Made in Occupied Japan" in English. This was then amended in December 1949 by SCAPIN 2061 to allow "Japan" or "Made in Japan" to be used instead (although many products would continue to use the Occupied Japan marking until the end of the Occupation in 1952).

It's a pretty common mark as porcelain was one of Japan's primary exports in the early postwar period.