Karat Markings on Old Rings

We have been debating for quite a while on when they first started stamping rings and jewelry with the K mark... 14K 18 K etc.. we are a group of metal detectorists in the states, that often find copper or brass wedding bands that are marked 14K or 18K, surely they were plated at one time and used to dupe the public into thinking they were buying gold, but we never could find anywhere that actually states when these marks were first used to date them. I know that most old UK gold was hallmarked.. but in the US do you know when the K stamp was first used?