Is there a stone in the ring? If there is, then it is likely that the die used to imprint the mark was not at an even pressure when it stamped the ring or a bit of dust or dirt got in the way when the ring was stamped. Therefore, it could actually be CZ.
If there is no stone in the ring, then the 7 likely refers to a size. I don't know what the c would stand for
Aren’t Champagne or Cognac diamonds color graded by the Chart: from C1 (lighter shade of Champagne) through to C7 (the stronger cognac color)?
Only C7 I know
Argyle Diamonds in Australia, a fancy colored diamond mine-known for pink and brown stones, uses this grading scale, ponpir. Possibly others as well.
But whether it's applicable to this ring would depend on whether the stone is a fancy color diamond. And in a ring from Korea, I think that's somewhat unlikely. Besides why would the setting be stamped when it's the diamond with the grade? Good brainstorming, though.:-)