These coins were struck at West Point as indicated by the (W), but they do not actually have the W mintmark. Their mint of origin is determined by the strap on the monster box they were sealed in from the mint.
When the mint sends out sealed monster boxes of (500) Silver Eagles, they strap tie the box together with a seal that specifies the mint where the coins were struck. San Francisco issues the boxes with a yellow strap with the words "San Francisco Mint," and the West Point mint issues the boxes with a white strap with the words, "West Point Mint."
Both NGC and PCGS recognize this as proof of where the coins were minted even though the actual coins have no mintmark. If the boxes are submitted sealed to the grading service, they will place the mintmark on the label in parentheses to indicate that the mint of origin is implied and does not appear on the coin itself.