If rare stamps were rock stars, the Inverted Jenny would be the hottest ticket on the concert circuit these days - and the only fans in line would be millionaires.

Two of the fabled airmail stamps, long considered the "holy grail" to collectors, because of their famous depiction of an upside-down Curtiss JN-4 airplane, will be sold at separate auctions in Jenkintown and in Baltimore today for hundreds of thousands of dollars each.

The 1-inch-by-3/4-inch stamps sold for 24 cents each when issued in 1918 to commemorate the first airmail flight between Washington, Philadelphia and New York.

Today, they are trading like hot stocks in a collectibles market that has heated up as the nation's economy has cooled. A near-flawless Inverted Jenny sold for just under $1 million last month in New York.

"The high prices being paid in the past few years is bringing them out of the closet," said Michael Schreiber, editor-at-large of Linn's Stamp News in Ohio. "People are selling them."

With fewer than 100 in known circulation (only a single sheet was ever released to the public), these "inverts" are to stamp collectors what the 50 known Honus Wagner Tobacco cards ("T-206") are to baseball enthusiasts: a crowning acquisition in limited supply.

"It's not the kind of thing you pull out at a bar to impress a woman. It's not a status thing," said Scott R. Trepel, president of Robert A. Siegel Auction Galleries Inc., of New York, which auctioned the Inverted Jenny for $977,500 in November. "It is what we knew when we were kids. It's the stamp we all wanted."

In the last two years alone, experts say, the market for high-end, rare stamps has reached levels not seen since the economic crisis of the late 1970s drew investors into stamp trading as an alternative to the stock market and real estate.

A demographic blip, however, is also at play in today's market surge: Many of those same investors are baby boomers who were avid stamp collectors as children.

Now, collectibles experts say they are seeing affluent near-retirees searching for high-yield investments. As such, they have rushed back to their childhood hobby with time and wealth to spare, and in large numbers befitting their generation.

"I think it always starts as a hobby," said John Apfelbaum, president of the Earl P.L. Apfelbaum Inc. auction house in Montgomery County that will sell one of the stamps today.

"And then, at a certain point when you start spending $50,000 or $60,000 on something, you start thinking to yourself, I hope this becomes an investment as well."

The Inverted Jenny being put up for bid at the auction house is part of a private collection of every U.S. stamp ever sold across a Post Office counter, Apfelbaum said. The collection will be auctioned in various lots.

The owner, who wishes to remain anonymous, fits the profile of today's high-end collector: He is a retired real estate investor from Maryland and avid stamp collector, whose family once had textile interests, Apfelbaum said.

His invert is less valuable than the one sold in New York because it has more flaws and is less-well-preserved. It could fetch $250,000 or more.

Apfelbaum, one of the nation's most well-known stamp dealerships, has sold an Inverted Jenny only twice before: in 1967 for $9,200 (today's equivalent of $58,000) and in 1990 for $90,000 ($144,000 today). The stamp dealer has been a family operation since 1930.

"It's kind of exciting for us," said Apfelbaum, a Cheltenham Township native, who runs the stamp firm with all three of his siblings.

The Inverted Jenny has long been popular because of its extraordinary autobiography - a tale filled with luck, wealth, drama and even a Roaring '20s spin on celebrity smut.

"It's been the most popular, valuable, sought-after United States stamp since it was issued," Apfelbaum said. "It's always been at the very, very top tier. It's kind of like a Van Gogh, really."

William Robey, a stamp collector, bought the sheet of stamps in May 1918, on his second trip to the Post Office in Washington. The first time, he was handed a sheet whose quality he did not like. He came back a few hours later and was handed the sheet of inverts.

"He looked at it, and he's like, 'Hold it, major problem here, that the plane's flying upside down,' " said Ken Martin, deputy executive director of the American Philatelic Society, in Bellefonte, Pa.

Robey could not believe his great fortune; stamp collecting was very much in vogue at the time. He knew he had landed a major score, and for just $24.

Soon after, postal inspectors visited Robey's house. They wanted the misprints back. He refused to turn them over.

The next week, he sold the sheet for $15,000 to noted Philadelphia stamp dealer Eugene Klein.

Klein resold the stamps to "Colonel" Ned Green, the obese and one-legged son of Hetty Green, known as the "Wall Street Witch." The heiress was so frugal that her son lost a leg to gangrene because of a lack of medical treatment.

"When he inherited her wealth when she died, he was the exact opposite," Martin said. "He was driving around in a limousine to New York City stamp shops."

The Green family's exploits were closely followed by the public.

"He had this glamorous kind of life," said Apfelbaum, who as a University of Pennsylvania graduate with a history degree has long found stamp stories fascinating. "He was like a Britney Spears of his day."

Green had Klein mark each stamp on the back to brand it and enable its future tracking. Green then began to break apart the sheet and sell the stamps as smaller pieces. This, more than anything, set in motion their enduring market ascendancy.

The stamp being auctioned here today has had five different owners, Apfelbaum said.

The surge in today's demand for these stamps has caught even dealers by surprise. Trepel did not expect more than $400,000 for the Inverted Jenny he sold last month for nearly $1 million.

"Two guys really wanted it," Trepel said. "We had three or four hands up over $700,000."

Stamps are not the only collectible experiencing a price surge. The wealthy are also flocking to antiques. So says Sebastian Clarke, of Freeman's Auction House in downtown Philadelphia.

"Whether it's an exceptionally rare stamp or it is a fine piece of furniture or a painting which has not been on the market before," Clarke said, "the prices are just going through the roof."

Contact staff writer Maria Panaritis at 215-854-2431 or mpanaritis@phillynews.com.