And what is the Holy Grail, you might ask? It's this October 1912 issue of ALL-STORY, containing the story "Tarzan and the Apes."
It's all but impossible to find a copy of this issue anymore, and when they are found, they can go for tens of thousands- it's not usual to see over $50,000 paid for a good copy. It might be safe to assume that it's so rare, that this scan, which I pulled from Galactic Central, is probably a scan of a photo pulled from a book.
Walker Martin announced to the world yesterday that now he has not one copy of this issue, but now he has acquired a second copy.
His story as to how he managed to have two copies is over at the Mystery File. Congratulations, Walker.