1870-S Three dollar gold coin up for auction -- hold on a minute!

An 1870-S three dollar gold coin has been put up for auction by the Four Seasons Auction Company.  The 1870-S coin was thought to be unique and struck for the purpose of placing in the cornerstone of the San Francisco mint.  The mintmark was also unique in that it was handmade, rather than stamped.  The coin last sold in 1982 for $687,500.

The current coin is a different 1870-S three dollar gold coin.  This should cause us to pause -- so this coin is perhaps not unique.  The mintmark looks different from its sister.  Does this mean that a second die was made just to produce a single coin?  Or is this story quickly losing its credibility.

The coin was apparently discovered embedded in a souvenir book in San Francisco.  How and why did it get there?  This remains shrouded in mystery.

The auction company expected the coin to fetch about four million dollars.  And yet the coins has not been authenticated by any service.  Again, this should cause us to pause.  The auctioneer encouraged people to view the coin in person and to bring experts.  But who is the expert who can authenticate a coin with a unique mintmark that was struck from a once-used die?

The coin has apparently been suddenly removed from the auction.  Either way, I encourage my millionaire friends to exercise caution.