The 1864-S is the rarest eagle from the San Francisco mint. Only 2500 were produced and my best estimate is that around 25-30 are known. Unlike its cousin, the very rare 1864-S half eagle, the 1864-S eagle is unknown in Uncirculated and I have personally seen only two that I regard as AU - The Bass lll: 658 (graded AU55 by PCGS: it sold for a remarkably cheap $36,800 back in May, 2000) and a coin owned by a West Coast specialist.
This is a generally well-produced issue although most have weakness on the radial lines of the stars. I have only viewed a small handful of 1864-S eagles that had any mint luster and most are heavily bagmarked. This was an issue that saw considerable circulation and the majority of survivors are very well worn.
It is my suspicion that someone is hoarding lower grade 1864-S eagles. There has only been one example sold at auction in the last five years or so, despite a combined population of twenty-three coins at PCGS/NGC. Even assuming that this number is inflated by resubmissions, my instinct tells me that some savvy collector is sitting on a group of five to ten 1864-S eagles and torturing those of us who would love to buy an example.
Between 2005 and 2011, I put together a complete set of $10 Liberties for personal challenge and pleasure. I worked very diligently on the set and in fact put together one of the finest sets ever assembled, better than the sets of Harry Bass and Louis Eliasberg, and better than the set in the Smithsonian. The 1864-S was the last coin I found for my set!!!!!!!!!!!!!! I searched forever, just couldn't find one. The day I finally bought a specimen awas the day I finished my set. And note that I was only able to find an XF45 grade coin.
There is one very well-heeled long term collector who has put together a complete set of U.S. $10 gold pieces, 1795 thru 1933. The last coin he obtained was the 1864-S $10 Liberty!!!! This one one very, very rare coin. And interestingly, it's not really that well known. Experts estimate that 22 to 26 specimens survive in all grades. I would say that this would definitely be the maximum number that could exist. In the very underrated $10 Liberty series, the 1875 Philadelphia is rarer with possibly a few as 8 to 9 known, but the 1864-S is definitely the second rarest coin in this underrated series.
Most 1864-S $10 Liberties are very well worn. I believe there are only 3 or 4 AUs known, the finest probably being a AU55.