EXPERTS

David Petruzelli, stamp expert with the New York City-based Philatelic Foundation.

Arthur Morowitz, president of Champion Stamp Company, in New York City.

RECOMMENDED RESOURCES
Scott 2003 Standard Postage Stamp Catalogue, edited by James E. Kloetzel, $45, www.amazon.com.

HOLY GRAIL
The 1918 "Inverted Jenny," which shows an upside-down biplane. Only one 100-stamp sheet was released before the error was spotted, so a single 24-cent stamp is now valued at upwards of $170,000. "Everybody hopes that someday they will stumble upon it," says Petruzelli.