Calibration check for expectations ....
The coin shown in the photos is a 1999-P ...... with a mintage of 688,440,000 .....
The silver planchet coins for this year/type were all minted in San Francisco ...... all in Proof strike with a mintage of 804,565.
Soooo .... IMHO ..... I find it hard to create a path whereby a single silver planchet from San Francisco made it's way to Philadelphia to strike a 'rare' coin.
Soooo ..... there is something else going on here ...... and we all await the voice of the experts.

David
Take a look at my other hobby ...
http://www.jk-dk.art