Jaime Hernandez: In the middle of April 2008, John Nanney from Leesburg, Georgia discovered the first known 2008-W Silver Eagle that had a Reverse design from the year 2007.
The official release date for all 2008-W Uncirculated Silver Eagles was on March 17, 2008. All 2008-W Burnished Uncirculated coins were offered directly to the public by the U.S. Mint, unlike non collector 2008 Uncirculated Silver Eagles which are only sold to authorized bullion purchasers by the U.S. Mint.
Sometime possibly as early as 2007, for some unknown reason the U.S. Mint decided to modify the reverse design of all upcoming 2008 Silver Eagles, including the Proofs. Consequently, all pre 2008 Silver Eagles should have a slightly different reverse design compared to regular 2008 and post 2008 Uncirculated and Proof Silver Eagles.
In the midst of transitioning from the old and to the new reverse design, the Mint inadvertently struck some 2008-W Burnished Uncirculated Silver Eagles (Uncirculated collector versions) with an old reverse design intended for pre 2008 Silver Eagles only. The two reverse designs are pretty much the same. However the letter fonts and portions of the design were considerably modified. Overall, it's fairly easy to distinguish between both reverses with the naked eye, especially since most of the lettering is now a different style.
Once the 2008-W Reverse of 2007 Uncirculated Silver Eagle errors surfaced, the U.S. Mint evaluated their coin inventory and discovered that at least 15 reverse dies from 2007 were employed to strike the reverse design of 2008-W Silver Eagles. Since the Mint strikes approximately 3,000 coins per each set of dies for the Burnished Silver Eagles, there are approximately 45,000 - 2008 Silver Eagles with a Reverse design of 2007 in existence.
The pick up point for distinguishing a 2008-W Uncirculated Silver Eagle with a Reverse of 2007, is on the letter U of United on the Reverse of the coin. The letter U on 2007 coins will lack the serif located on the right lower hand side of the U. In 2008 and going forward, all Uncirculated and Proof Silver Eagles should contain a serif on the bottom right of the letter U of United.