What’s been sucked up in a vacuum cleaner by Lucille Ball on “The Lucy Show,” found — then discarded — by Homer Simpson and used to mail a postcard in “Brewster’s Millions?” The Inverted Jenny, of course — a sought-after 1918 airmail stamp with a funny misprint: The biplane featured on it is upside-down. In the Smithsonian National Postal Museum’s brand-new William H. Gross Stamp Gallery — an additional wing to the museum — a block of four Inverted Jennys is now on permanent display at the museum for the first time. “It’s the stamp everybody wants to see,” says Cheryl Ganz, the museum’s lead philatelic curator. “It’s like our Hope Diamond.” Jenny sits alongside more than 20,000 other rare stamps in the gallery’s state-of-the-art display cases. “Right now, we get about 400,000 visitors a year,” Ganz says. “I wouldn’t be surprised if that jumped 50 percent or more.” Whether you’re a dedicated philatelist or you don’t even know the word (it means you dig stamps), here are a few examples on view you won’t want to miss.

The Inverted Jenny
In 1918, savvy stamp collector William T. Robey visited a (now defunct) post office in downtown D.C. and asked to see the new 24-cent airmail stamp. He knew there had been a rush to print the stamps in time for the first airmail flight and suspected there would be errors — a favorite feature among collectors. He was right: While leafing through the new issues, Robey discovered (and bought) a sheet of 100 stamps with a belly-up airplane.

He told fellow collectors, and word spread to postal inspectors. Keen to bury the embarrassment, the inspectors visited Robey at his home and threatened to confiscate the sheet. Spooked, Robey quickly sold the one-of-a-kind sheet of stamps for $15,000 to a prominent New York collector. Today, individual Inverted Jennys — all from the original Robey sheet, the only one known to have survived — can sell for more than $1 million.

Pony Express envelope
This envelope (above right) was mailed from California to New York via Pony Express in 1860. Along the way, the rider was probably killed by Native Americans, Ganz says. Someone later found the riderless pony with its mail pouch and forwarded the mail. By the time the envelope reached New York in 1862, the Pony Express no longer existed.

The letter inside, which has been lost, was probably business mail, since banks and other companies were the only ones willing to fork over the extra $5 per half ounce for Pony Express service. (The Postal Service’s cheaper cross-country rate sent your letter on a monthslong ship ride around South America.)

“When you think of how cutoff people on the West Coast felt from people on the East Coast, to get a letter in a few weeks was a big deal,” Ganz says.

1-cent Z-grill
There are many examples of the 1868 Benjamin Franklin stamp in circulation, but this one is special: If you flip it over, you can see a waffle ironlike imprint, known as the Z-grill. There are only two such stamps in existence: One is owned by the new wing’s namesake, William H. Gross, who got it in exchange for an Inverted Jenny block of four. The one now on display at the Postal Museum is on loan from the New York Public Library.

The Z-grill was an early attempt to keep people from reusing stamps by washing off the postmarks, Ganz says. The impression broke up the stamp’s paper fibers, making them more absorbent. About 1,000 1-cent Z-grill stamps were produced, but few survived, probably because they were used for junk mail. Now it’s the rarest and most valuable U.S. stamp around, worth about $3.6 million.

“You can’t have a complete collection without it,” Ganz says.

Postage Posterity

Each year, the U.S. Postal Service issues about 20 new commemorative stamps. According to a USPS survey, the most collected (bought but not mailed) stamps of all time, are:
Elvis Presley, 1993 (124 million) Voters overwhelmingly chose a rendering of young Elvis when the Postal Service proposed the stamp.
Wonders of America, 2006 (87.5 million) This set of 40 stamps trumpets America’s natural and man-made marvels.
Marvel Super Heroes, 2007 (85.5 million) Spider-Man, Iron Man and a hero named Sub-Mariner, who wears scaly underwear, are among the stars of this 20-stamp issue.

National Postal Museum, 2 Massachusetts Ave. NE; daily, free; 202-633-5555. (Union Station)