천자문
In the year 1742 special characters began appearing on the reverse sides of some "Sangpyeong Tongbo" cash coins, many of these special characters were used to indicate which furnace had been used to produce them or to which "series" they belonged. The series number may be to the left, right, or at the bottom of the center hole of the coin. The furnace designator may be either a numeral or a character from the Thousand Character Classic.