I recently found this 2000 Skybox Mike Piazza card with what I thought was a NSCC stamp, but it's not actually a stamp. Fleer/Skybox actually made these specifically for the National.
The card is basically the exact same as the regular 2000 Skybox Mike Piazza card with the only differences being the reference on the front to the 21st Annual NSCC at the Anaheim Convention Center July 24-30, 2000 and the numbering "6 of 6" on the back.
If my calculations are correct, this is my 597th different Piazza card. I was very happy to add this to my collection, especially from a 15/$1 box.
The other 5 cards from the set include Cal Ripken Jr., Ken Griffey Jr. Alex Rodriguez, Derek Jeter, and Mark McGwire. Along with the Piazza I was able to find a Mark McGwire. If you're interested in the McGwire let me know.