KP is an acronym for karat plumb. These letters signify a guarantee that the ring in question contains gold with a purity rating equal to or higher than the amount stamped, in this case 10 karats.
Because many markets allow jewellers up to half a carat of “wiggle room” when marking purity levels, the extra letter is employed. As a result, a ring that is marked as 10k may actually only be 9.5k pure. When compared to other gold products that merely have the conventional karat marking, the KP label denotes a higher level of purity.