Stamps are often nominated as the most expensive man-made objects on a value to weight ratio. Usually quoted as the world-record holder is a Swedish stamp, the Treskilling Yellow, discovered in an attic in 1886. It sold for $US2.06 million at a 1996 Geneva auction, then again in May 2010 for an allegedly higher, but undisclosed, price. The reason for its extreme value is that it was mistakenly printed in the wrong colour. It's the only example recorded in the world.
Nothing quite compares with this on the Australian scene but a few more modest world records were set in the Melbourne suburb of Boronia last month at a Prestige Philately auction.
What's remarkable is that these are not stamps in the traditional, post-office sense. These are known as revenue stamps, the ones marked ''stamp duty'' seen attached to old legal documents, notably title deeds. They record various denominations of stamp duty - up to £100 in some cases - and unused or specimen examples from the second half of the 19th century are now attracting serious money, more than $3000 for significant items.
''It's the only way for many collectors to get their hands on a stamp that is generally unused,'' the chairman of Prestige Philately, Gary Watson, says.
The latest batch of revenues was part of the sale of the Les Molnar collection of Victorian stamps. That's Victoria as in the Australian state, yet Molnar, a mathematics teacher and entrepreneur, accumulated these from his home in St Catharines, Canada.
The period of postal history after the Victorian gold rush is popular around the world and revenue stamps (sometimes called postals fiscals) proved to be the most successful section of the sale.
All stamps sold, most above estimate, and some two or three times above.
The £100 example sold for $1200 but that's nothing compared with an £8 version that sold for $3600. It scored higher because there are only two known in the world.
A 3d mauve example sold for $3800; its catalogue estimate was $1500.
We can assume that Molnar would have been happy with the results of his Melbourne sale. The auction cleared 88 per cent by volume, achieving total sales of $486,500. That's a handy super bonus for his retirement.
There have always been collectors of revenue stamps but these record prices are a recent phenomenon.
''We had a large collection of the Ultra High Values back in September 2011,'' Watson says. ''They virtually all sold. They are listed by Stanley Gibbons [the global authority on stamp values] because, technically, they could be used for postage between 1884 and June 30, 1900.''
The possibility that these may have been used for postal purposes makes them of interest to the majority of collectors who normally ignore revenue stamps.
But does this mean they are a good investment? Watson is cautious on this subject.
''Possibly so,'' he says. ''At some point, Gibbons have to get their act together and make a distinction between postally used, many of which are very scarce, and Cancelled to Order [CTO] from specimen sets, most of which are reasonably common.
''That would start to change people's perceptions. And if they were to price the high-value specimen stamps - the only way most of them exist is in an unused state - the prices could go through the roof.''
He admits that this shift in attitude could take some time but if it does, anyone with mint Victorian revenue stamps in their albums could be on a winner. The latest auction results indicate some collectors are taking that punt. Various overseas bidders competed online at the sale.
Not quite in the same league, other types of revenue stamps also have their followers. Beer duty stamps were produced in Victoria late in the 19th century, fixed to either bottles or labels. Molnar also included a small selection of these in his sale and one of the largest examples, for a one-kilderkin container, sold for $500.