The card is incredibly difficult to obtain, and usually sells for quite a bit more than the card in its typical stock. I've owned 3 of them - really regretted selling my first 2 (PSA 3 and PSA 5 I believe) - was really glad to have picked up the PSA 2 in the last Heritage auction. The pop report tells all - I think there are 6-8 total Gray Backs vs. 2600 or so regular backs.
Here's a comparison between the two:
