Notes:
In 1915, San Francisco hosted the Panama-Pacific Exposition to celebrate the opening of the Panama Canal.  To commemorate the event, the United States mint struck a variety of coins in silver and gold, including a Half Dollar, Gold Dollar, $2-1/2 Gold, $50 Gold Octagonal, and $50 Gold Round.  The coins were sold at the Exposition individually or in a variety of combinations and/or sets.  Apart from Patterns and semi-official Territorial gold coins, the United States had never issued a $50 gold piece.  Not sure of how many they could sell, Mint officials struck 1,509 Octagonal and 1,510 Round versions.  Because of the high cost of the coins and the sets, very few coins actually sold.  Of the 1,510 $50 Rounds, 10 were set aside for assay purposes, 483 were sold to the public, and the remaining 1,017 coins were destroyed.  The Panama-Pacific $50 Round ranks as one of the great numismatic rarities of the Twentieth Century. 

Images courtesy of Northeast Numismatics, Inc.

Varieties (2):
Octagonal - net mintage of 645 coins
Round - net mintage of 483 coins

Significant examples:
(See individual varieties)

Recent appearances:
(See individual varieties)

Sources and/or recommended reading: