In 1864, employees at the San Francisco Mint ramped up production of Dimes and stamped out 72,500 more units than in the preceding year. Apparently, this was all done with a single pair of dies, according to Seated Dime researcher Gerry Fortin.
Most 1864-S Dimes are struck well, but those from the latest state of the dies will show some graininess around the date and in other areas of the fields on both sides.
Mint State examples are scarce but not rare. The typical Uncirculated 1864-S Dime is MS64 -- in fact, the PCGS CoinFacts Condition Census starts at MS64 and tops out at MS67. Like the 1864 Dime, the 1864-S appears to be an excellent value in most grades.