Biged239, these are the characters for 'Republic of China', although you'll often find them in reverse order

and occasionally in different fonts as well. These will identify Chinese stamps issued between 1910 and 1949, including stamps issued for and in the Japanese-occupied areas of China.
Sorry I can't give you a Scott number for this stamp, but it was part of the January 1946, Chinese National Currency (CNC) surcharges.