A dark red octagonal piece of paper about as big as a Susan B. Anthony dollar -- and already, for its size and weight the most valuable object on earth -- sold in New York for $850,000 this weekend.

More than 1,000 investors and collectors, who tended to be still-faced, sleek and plump, with conservative suits, gold jewelry and horn-rim half-glasses, packed the Waldorf-Astoria's grand ballroom to bid on 424 stamp rarities in a four-hour session held by Robert A. Siegel Auction Galleries.

But it was the sale of the one and only 1856 British Guiana One-Cent Black-on-Magenta stamp, for which the seller, Irwin R. Weinberg of Wilkes-Barre, Pa., and his nine-member syndicate paid $280,000 a decade ago, that everyone had come to see.

Siegel, a 50-year veteran auctioneer, said it was the greatest stamp auction in history, bringing in $5,128,000 for an average of more than $12,000 per stamp. With a 10 percent commission and another 10 percent premium from buyers, Siegel made a million dollars in four hours.

He also lost his voice.

"The bidding was done by holding up numbered paddles," he said just after the sale, "but a few special customers had made arrangements with me privately to signal by turning their head or tapping their nose or some such. I had two people who were doing this for the British Guiana, but one of them never even made a move because it went over his limit so fast."

The bidding started ata $325,000 and in a matter of seconds passed the $700,000 mark.

"There were quite a few bidders at first, but they thinned out pretty quick," he added. When he reached $850,000 there was silence -- some experts had expected the stamp to bring more than a million -- and then, in the awed tone reserved for statements of great moment, he announced the stamp "sold -- to a collector who prefers to remain anonymous." The new owner will, however, exhibit the stamp, as did Weinberg.

For some minutes decorum broke down as TV cameramen rushed desperately from one likely-looking bidder to another. They shot film of one man in the second row who appeared to be wearing a secret smile. But, shrugging in the British fashion (with the mouth corners only), he denied that he was The One.

It was an astonishing afternoon for the stamp world. Investors were climbing all over each other to pay unheard-of prices for what is sure to become a far more popular investment medium than ever before.

A Canal Zone block of 10 with a printing error, catalogued at $55,000, went for $130,000. A charming sample of the famous upside-down airplane stamp sold for $125,000. An inverted flag, "considered the best copy known of this rarity," was catalogued at $30,000, but the bidding started at $62,500 and went up to $70,000 in about 20 seconds.

And a prime Bermuda Perot, one of those stamps signed by a provincial postmaster in the same manner as the British Guiana, and also unique in being the only such stamp existing on an envelope, or cover, started at $65,000 and rocketed to $210,000. Kings and Schoolkids

What is it about stamps? Why is stamp collecting by far the world's most popular hobby, its addicts ranging from the powerful -- Franklin Roosevelt and King George V come to mind -- to children in grade school?

"First, it's an investment, of course," said Weinberg, who brought his family to New York for the occasion. "I bought the magenta as a hedge against inflation in 1970. At the time rare postage stamps weren't especially considered a good bet, but I knew that eventually they would be accepted as an international commodity: They're portable, liquid and finte."

He kept it in a bank vault, but over the years he enjoyed taking it -- in a briefcase locked to his wrist -- to exhibits around the world, from Prague to New Delhi.

"People flock to it. There's something about its smallness and its being the only one.Kids always want to see it. I began thinking about this in '68, when I knew that there would be heavy inflation with the end of the Vietnam war, and I believed the safest place to be was in a commodity where an international market existed. At the time I didn't even know the stamp was about to become available. We were lucky."

He has been buying and collecting stamps since he was 15. Before he was out of high school he had built up a mail- and phone-order business.

"I was a Fuller Brush salesman for a while," he said, "and I came to New York, but then I realized my stamp business could be done anywhere, so I went to home to Wilkes-Barre.

"Of course there's the monetary reward, but there's also an esthetic element if your're sensitive to owning fine things.There's the thrill of ownership, I guess, the squirrel instinct. I hope the next owner will be as interested in taking it around as I was. I'll miss it. But I've made my point. I've shown the value of stamps as a weapon against inflation.

He said he had no idea who the new owner is. This nameless individual, who actually paid $935,000 for the stamp, including his 10 percent buyer's premium, joins a very small and very special club.

The stamp was issued April 4, 1856, at Demerara, now Georgetown, in British Guiana, now Guyana. Running out of stamps from London, the local postmaster had some provisional stamps printed in the emergency.

The printers used ordinary type and a small ship design that normally headed the shipping-news column in the Demerara newspaper. The motto "Damus Petumus Que Vicissimc ("We Give and Seek in Turn") was added to the words "British Guiana," "Postage and "One cent."

To protect from forgery, assistant postmaster E. D. Wight put his initials on the stamps.

One of them surfaced in Demerara in 1873, when that most celebrated of obscure schoolboys, L. Vernon Vaughan, found it among family letters in his attic. More interested in other stamps, he sold it to a collector named McKinnon for six shillings. McKinnon sold it for $600 to Thomas Ridpath, a Liverpool dealer, and Ridpath sold it for somewhat more to Count Philippe von Ferrary.

When the count's estate stamp collection was auctioned in Paris in 1923, the bidding for this single stamp ran high.

It was rumored that an agent for George V was bidding, since it is the only Commonwealth stamp missing from the royal collection. In any case London dealer Hugo Griebert bought the magenta for $32,500 for Arthur Hind, a plush manufacturer who lived in Clark Mills, near Utica, N.Y.

Hind had been prepared to go to $60,000 for the stamp, he said later. According to tradition, a second magenta was found in 1929 and sold to Hind for a large amount. Hind is supposed to have set it afire with his cigar and muttered, "Well, there's still only one."

Hind died in 1933. His widow sold the stamp to an unnamed collector for an amount said to be more than $50,000. In 1970 it came on the market again, in the hands of Siegel Galleries, and it was then that Weinberg bought it. The Allure of Rarity

"It's not easy to put on a sale like this," sighed Robert Siegel, exhausted at the end of a session that more than doubled the previous record of $2 million in his 1979 Rarities of the World auction. "I mean, it's easy to sell them. What's hard is finding so many stamps of this quality."

The appeal of stamps goes back a long time. Hardly a month after the first gummed postage stamps were issued in 1840, people were collecting them.

Many start young, filling a modest album with stamps bought in packaged assortments. Later they become investors, and still later the investors tend to turn back into collectors, specializing in one country or era or type, often refusing to sell their favorites at any price.

There are so many kinds of collectables: stamp precursors, like the 1454 letter from Florence now in a British collection, or the early products of Col. Henry Bishop, probably inventor of the postage stamp in 1660.

There are stamps from shipwrecks, battlefields, expeditions. There are the stamps with mistakes (recalling the excruciating story of a man who bought a sheet of 15-cent stamps, noticed that the centers had been inverted, and indignantly turned them in for a normal sheet . . . those inverted stamps fetch five figures today) and stamps that were used as currency in the Civil War or other wars, when coins customarily are hoarded.

There are the stamps used by Jews escaping Nazi Germany, undetected where gold or jewels often were spotted and confiscated. There are the lengendary 1847 Mauritius rarities, so sought after that a magazine is devoted exclusively to them.

There are British stamps overprinted with swastikas by the Nazis for use in the Channel Islands in World War II. Not to forget the Penny Blacks and Two Pence Blues of 1840, a classic collector's item because they range from run-of-the-press (68 million Blacks were printed, and 6 million Blues) to superb single stamps with perfect margins and neat cancelations.

"Europeans have been investing in stamps for three generations," said Weinberg, "but until recently Americans tended to trust the dollar and not get into commodities. It's a different story today."

The very last item sold Saturday was a set of gold replicas of several famous stamps, including the magenta. It was knocked down at $3,250. The buyer was Mrs. Irwin Weinberg.

"It's such a pretty little thing," she said. "I just couldn't resist." She beamed with the thought of it. CAPTION: Pictures 1 through 3, Irwin and Jean Weinberg during the auction, the $850,000 stamp, U.S. $5 deep green and black issued 1917-20. Photos by AP