In 2017, the public discovered that the U.S. Mint had secretly minted a small number of Silver Eagles at the Philadelphia and San Francisco Mints because of the Freedom of Information Act.
Only 79,640 of the nearly 50,000,000 2015 Silver Eagles were mined in Philadelphia. Well, that's about 1/10th of a percent! This is the rarest Bullion Strike Silver Eagle in terms of minting.
Keep in mind that many of the sealed boxes that were minted at the Philadelphia Mint were opened before this information was released in 2017, which means that they will never be classified as a Philadelphia coin, which adds to the rareness of this piece.