Large blue and white charger

by Rich C.
(Lancaster PA. U.S.A.)

This large plate was acquired by my mom in Indonesia just after WW2 and brought home in 1949. I don't know anything about it. Can someone let me know whether it's Chinese or Japanese, the approx. age of it, and what the 2 square marks on the back means? One mark is painted and the mark right next to it is inscribed. They are both under the glaze. The plate is 14.5 inches in diameter and is thick and heavy. Thanks, Rich