One of the prime rarities of the US gold coin series is being sold in Heritage's 2009 September Long Beach Auction.

Half a million double eagle coins were produced in San Francisco in 1920. However, unlike later dated coins, 1920-S $20 MS64 PCGSs were never hoarded or replenished, and virtually the entire mintage was destroyed during the gold recall of the 1930s.

Unlike its Philadelphia made equivalent, the coin is rare in all grades.

"The 1920-S is one of the prime rarities in the Saint-Gaudens Double Eagle series, if not in the entire 20th century U.S. gold series," says numismatist David Akers in his Handbook of 20th Century United States Gold Coins.

"In fact, it is the rarest collectible Saint-Gaudens Double Eagle after only the 1927-D, 1930-S and 1921."

The 1920-S is actually rarer than the 1927-D and 1930-S in high grades.

High-grade examples, of which there are very few, were almost certainly obtained at the time of issue by collectors who had close ties to Mint or other government officials.

The coin will appear as part of the 'Vegas collection', so-called because its consignor's numismatic hobby flourished during the economic boom in Las Vegas.

Online bidding for the auction is now open on the Heritage Auctions website.