Stamps with grills are the most difficult challenge for anyone assembling a complete U.S. stamp collection. (Grills are indented patterns, meant to soak in cancellation ink to prevent reuse of the stamp.) In the early 1900s, philatelist William Stevenson assigned letters to the different grill patterns.
The Z grill was used very briefly, so it’s hard to find. Just two copies are known of the 1-cent and 15-cent Z grills, making them the rarest U.S. stamps. In Miller’s lifetime, no one knew the 15-cent Z grill existed. His 1-cent Z grill was the first one ever identified. It is displayed here together with the other copy for the first time.