1999 Platinum American Eagle Proof 70 Set Description:
The United States Congress authorized the U.S. Mint to make .9995 platinum coins in 1996. For each year from 1997 to 2008, the Mint made 4 coin sizes: 1oz ($100 face value), 1/2oz ($50), 1/4oz ($25) and 1/10 oz ($10). There are many fewer proofs than mint state coins minted each year and only a very small percentage of those coins receives a Proof 70 certification.
Source: Mintage figures provided by the United States Mint. Proof 70 Population indicates the maximum possible population of perfect condition sets or individual coins for the year given (NGC, December 2021).