40 years in the business, and I've never heard of "10KM". If that is a stamp you see, I would guess that the "M" is a manufacturer. If it weighs 21.5 grams and is solid 10K, you have a piece there that has a 100% of spot melt value of over $550 today. Back when our store was open, we would have paid a seller about $475 for it, again assuming 10K. Worth getting it checked by someone who can tell you for sure.
