Certain serial numbers on the that the mint was forced to give out by the right of information act or something like that. So that's how they determined that there were only the 79,000 or so minted from Philly. So NGC ANACS. and PCGS took those Serial #d boxes and were able to put Philly on the slabbed description. There are only a few hundred with a grade of 70 and they have been selling for around $3,700 to $5,500. I would sel mine in the lower range if anyone is interested or trade. Thanks for all the responses, Torrey