It was one of the most tumultuous years in U.S. history.

Opposition to the Vietnam War was reaching a crescendo, two American heroes -- Robert Kennedy and Martin Luther King, Jr. -- were assassinated and Richard Nixon was elected as president. With all of this going on, it's easy to understand why most people weren't thinking about baseball cards in 1968.

"We've got the Vietnam War taking place and there was a riot in Chicago, with students protesting at the Democratic National Convention. The card market, at that time, had to be about as exciting as watching golf," said renowned hobbyist Marshall Fogel. "The culture had changed because people had found other interests."

It didn't help that during this period Topps was producing very mundane cards, said Fogel.

"If you look through all of the sets of the 1960s, especially 1966 and 1967, they're posing (in the card photos)," he noted. "They're not very innovative. You know why? Because they (Topps) didn't care; they had an exclusive market."

Fogel believes that in 1968, Topps realized they had to do something to re-spark interest in their product, and, as a result, they created the revolutionary 1968 Topps 3-D Set.

"I think this (the 1968 Topps 3-D set) represents an attempt to recover. And I think when they (Topps) made the attempt to recover, they drove a Mercedes into the garage. These cards are spectacular," said Fogel, who owns the No. 4, 1968 Topps 3-D set on the PSA Set Registry.

The 12-card, 3-D set is considered one of the hobby's rarest and most coveted test issues. Measuring 2-1/4" by 3-1/2", these unnumbered cards boast round corners. The fronts include the player's team, name and position and showcase a player against a blurred background. A thin layer of plastic produces a 3-D effect when the card is tilted. The regular backs are white and blank.

These cards were sold in packs in a small number of Brooklyn stores, says longtime dealer Stan Martucci. Approximately the same size as the regular card, a white cardboard easel to display cards also came in at least some of these packs, says Robert Lifson, president of Robert Edwards Auctions.

"It (the easel) had black and white instructions on how to punch it out and it just allowed you to stand it up," said Lifson.

Fogel and Dave Hornish, a Topps test issue enthusiast, agree that the reason these cards were not produced in higher quantities was likely the cost of making them.

A number of variations of these cards have also been uncovered. Disparities in the background photos and the cropping of the photos on these cards have been noted. Proof cards of Rick Monday, John O'Donoghue and Tommy Davis -- that did not include any text or logos -- have also surfaced and were auctioned off for more than $25,000 each in a SCP Auctions/Sotheby's sale in June 2006. A rare Brooks Robinson prototype has also turned up.

1968 Stottlemyre

"It looks like an upside down '69 Topps (card)... with the Orioles (team name printed) on the top," claimed Hornish, who believes this card would fetch at least $50,000.

More common variations of these cards boast a stamp on their backs. The stamp reads, "This is an experimental XOGRAPH card produced as a limited edition. Not for public circulation or distribution. Not for resale. To be returned to: Visual Panographics, Inc. 488 Madison Avenue, New York, New York."

The scarcity of these singles is one of the reasons they are coveted. Another attraction is the size of the set. Well-known hobbyist Charlie Merkel, who owns the registry's No. 1 set, purchased his cards at a Mastro Auctions sale.

"I've done several of the smaller regional sets... Number-wise, they're small enough that you can put them together in really, really high quality without having an interminable amount of time taken," he said.

The revolutionary design and aesthetics of these cards also appeal to collectors.

"Other companies never could duplicate what they did with the (1968) Topps 3-D set. It's almost like you could climb in the picture, they're so three-dimensional," said Fogel.

The respected hobbyist says it's a combination of factors that inspired him to put this set together.

"One, it's, in and of itself, beautiful. Two, the cards are of great quality. Three, it's got (Roberto) Clemente in it," said Fogel.

Considered by many to be the best card of the 1960s, the Clemente is the highlight of this issue. In December 2006, a PSA 10 example sold for $30,343 in a Mastro Auctions sale. Of the 29 Clementes that have been graded by PSA, six have received a Gem Mint grade.

"It's not one of the tougher cards," said Merkel. "There are actually quite a few of them."

The set's only other Hall of Famer is Tony Perez. Of the 37 Perez cards sent in, just one has received a PSA 10 grade. A PSA 9 Perez lists for $3,500.

Minor stars include Curt Flood, Boog Powell and Rusty Staub. Powell is widely regarded as the most difficult card to find in high-grade. Hornish recalls seeing a Powell on eBay that was in "VG" condition sell for almost $1,000.

Staub is the sole regular card that does not have a PSA 10 example. It's also the only single that Merkel does not have in PSA 10 condition.

1968 Powell

"I constantly look for it... You just don't see many of these offered," said Merkel.

Scratching and cracking are the primary condition issues with these cards. Merkel says the enamel on the card fronts tends to crack when exposed to heat or sunlight. Lifson has also noticed these condition issues.

"Cracking is definitely an issue," said Lifson. "They scratch and they crack easily."

In general, however, Merkel is amazed at how many of these singles have been graded as PSA 10s.

"If you look at the population on this set, the percentage of (PSA) 10's is amazing (about 12 % of the submissions have received a Gem Mint grade). The relatively small issuance of these went to people who more or less stuck them away," he said. "So, while there's not many of them out there, what there is out there is pretty high quality, relative to most sets."

It's this scarcity and quality -- coupled with the revolutionary design of this set -- that explains the continued interest in it nearly 40 years after its release.

"It's really special because it's surrounded by bridesmaids. This is the bride of the 1960s," said Fogel. "Topps remembered how to make a great card. This set stands not alone, but really in the family of the artful cards of the 1950s."