There are 10 and they can make fortunes. When a package of 10 Penny Blacks is auctioned all hell will be let loose in the stamp-collecting world.
SOME STAMP collectors would give their right arm - or, perhaps, an estimated pounds 250,000-pounds 300,000 - for this envelope bearing 10 Penny Blacks, to be auctioned by Stanley Gibbons on Wednesday.

It has everything going for it. It is the largest known multiple block of penny blacks ever found on a cover. And covers are hardening in value now that sophisticated collectors are after "postal history" rather than stamps removed from the covers that were their historical context. Moreover it is a first-day cover - posted on the penny black's first day of issue, 6 May 1840.

But by far the biggest attraction is the penny black itself. It is not, as some people think, the rarest stamp. About 68 million were issued in the year before it was superceded by the penny red. But as the first postage stamp in the world, it will continue to command pride of place in any collection - and to lure investors.

As little as pounds 25 will buy a used one in passable condition at auction. Now is the time to stockpile them, and the better the condition, the better the investment.

In the West, today's children may prefer computer games to stamps, but thousands of newly rich adults in Asia and the Far East, particularly China and Hong Kong, are beginning to collect. They are all going to want their penny blacks.

At present, the Far Eastern economies are pausing for breath. And penny black prices have risen by only 5 per cent a year compared with 10 per cent for stamps as a whole, a sign that the first surge of newly rich collectors in those countries have already acquired their token penny blacks. But see what happens when they discover how many different varieties of penny black they can buy.

Investment in stamps has acquired a bad name since the boom and bust of 1979. In November of that year a speculative price spiral, accelerated by over-the-top values in Stanley Gibbons' 1980 catalogue, culminated in crazy prices being bid at Sotheby's auction of the Vaduz collection of British and Empire rarities. The following day the market crashed as speculators vied to cash in their collections and dealers came to their senses.

Speculators need to feel fenced in before they resort to buying stamps. It requires the economic conditions of the late Seventies - a stagnant property market, rising inflation and a scary stock market - before those get-rich-quick advertisements for stamps start to appear in newspapers.

Today, though property is dull and inflation is looming, shares have looked a better bet than stamps. There is no rush to buy stamps, although prices have been recovering steadily since Vaduz.

Those who do invest are now more sophisticated, and a forthcoming publication by Stanley Gibbons will fascinate them and help to revive the market. It is a reprint of Charles Nissen's plating guide to penny blacks of 1922, at present worth pounds 400-pounds 600 secondhand.

Here we plunge into the arcana of stamp collecting - the secret knowledge that turns collecting into an obsession. During the short life of the penny black the printing presses wore out 11 metal printing plates for the sheets of 240 stamps (12 if you count the retouching of plate one, which printed 10 million of them). Later plates printed fewer stamps, making penny blacks printed from them rarer and more valuable. But how do you tell from which plate your penny black came?

The clue is the stamps' letters in the two top corners. They are in sequence along the rows: AA, AB, AC and so on - a device to deter forgery. Each letter was punched into the plate by hand, whether by one man or a team is not known. What is known is that whoever punched them had off days - a tiff with the wife, perhaps - so that some letters are ham-fisted, lop-sided or off-centre. These tell-tale variations will link each stamp to its plate - if only a key to them can be found.

That was Nissen's task. Using dated postmarks as a guide, he amassed enough penny blacks to reconstruct all 11 plates. His reconstructions were sold by Stanley Gibbons but his illustrated book survives. Armed with a reproduction of it, and a copy of Stanley Gibbons' Queen Victoria catalogue, which lists different values for penny blacks from different plates, collectors will be able to tell how much their penny blacks are really worth.

Example: a fine or very fine used penny black from plate 1a is valued at pounds 220 by Stanley Gibbons while an almost identical one, from plate 11, the last penny black plate, is valued at pounds 1,600. This is a potential treasure trove for those with spare time, spare cash and a magnifying glass - especially retired people who used to collect in their youth. And Far Eastern collectors, still mistrustful of Western ways, tend to trust catalogues.

A reconstructed sheet of penny blacks - a collector's dream - was sold for pounds 15,645 last year by Harmers, the London stamp auctioneers. Plate 11, incidentally, printed only 168,000 of the penny black before the penny red replaced it.

The new stamp was issued because rogues found that they were able to wipe off the Penny Blacks' red Maltese cross cancellation and resell them as unused. Black cancellations on red stamps soon put a stop to them.

There are plenty of other varieties listed by Stanley Gibbons. Double letters - the sign of a punch-drunk engraver? - will raise the value of a pounds 150 used penny black (plate unspecified) to pounds 200, more if the plate is known. There are also premium values for inverted watermarks, guide lines in the corners and "profile clear" - specimens whose cancellation has not obliterated Queen Victoria's head.

A block of four penny blacks is highly collectable - catalogued by Stanley Gibbons at a minimum pounds 3,250 used. And, of course, unused specimens carry a big premium: a pounds 150 used one would be pounds 3,000 if it were unused, a block of four pounds 14,000. It still pays rogues to remove cancellations and expertly reapply gum. So it will pay you to ask for a certificate of authenticity from a reputable dealer, the Royal Philatelic Society or the British Philatelic Association.

You can still pick up damaged penny blacks in street markets for under pounds 10. These are the ones with margins missing, the product of those nights when the postmaster sat up late with a pair of scissors, cutting the unperforated sheets by gaslight.

Ignore the damaged and the grubby. Better to invest pounds 100 at auction in a fresh-looking profile-clear, or more for a block of four - or one on a cover with a May 1840 postmark or a regional Maltese cross cancellation listed in the catalogue. You will have to do hours of homework before you can spot varieties and outwit the trade. Hours of fun, as they say.

Stanley Gibbons, 399 Strand, London WC2 (0171-836 8444). Harmers, 91 New Bond Street, London W1 (0171-629 0218).