The letters KP stand for karat plumb. These letters indicate that the ring in question is guaranteed to contain gold that has a purity grading equal to or greater than the amount stamped, which is 10 karat in this case.
The additional letter is used because many markets give jewelers up to half a karat of "wiggle room" when marking purity levels. This means that a ring marked as 10k might only have a purity rating of 9.5k. The KP mark indicates a product that has a higher purity level than other gold products that have the standard karat-only marking.