This is a spectacular Clark Gruber and Co. double struck $20 gold piece. Clark Gruber was the most respected and well known financial and banking company in the pioneer and Gold Rush days in Colorado. Clark Gruber and Co. opened a bank and mint in 1860 and became coiners, striking their own gold coins of proper weight. The denominations struck were $2 1/2, $5, $10 and $20. These pieces were accepted as currency since Clark Gruber and Co. was a well known private minting firm with an excellent reputation. Although PCGS certified this double struck Clark Gruber $20 as EF 45, in my opinion it is closer to AU 50 since it exhibits some luster and has the detail showing. Since it is double struck with some rotation between the two strikes, it is a spectacular and rare major mint error. It would be the centerpiece of any U.S. error collection. There are 2 or 3 known Clark Gruber $20 gold pieces struck over Assay pieces, but that was done deliberately and is considered an overstrike and not a mint error.