From Making Money where Moist is observing Ankh-Morporkians bidding for one his first issue $1 notes.

Anyone remember the Triangular Blue? said another bidder. Fifty pence, it cost. I put one on a letter to my aunt; by the time it got there it was worth fifty dollars! And the ol` baggage wouldn’t give it back!
It’s worth a hundred and sixty now, said someone behind him. Auctioned at Dave`s Stamp and Pin Exchange last week.
Stamp collecting! It had started on day one, and then ballooned like some huge . . . . thing, running on strange, mad rules. Was there any other field where flaws made things worth more? Of course, when Moist has spotted this he had put in flaws on purpose, as a matter of public entertainment, but he certainly hadnt planned for Lord Vetinari’s head to appear upside down just once on each sheet of Blues. One of the printers was about to destroy them when Moist brought him down with a flying tackle.
But the stamp collectors . . . . they believed in small perfections. It was possible to get one small part of the world right. And if you couldnt get it right. You knew at least which piece was missing. It might be, f`r instance, the flawed 50p Triangular Blue, but there were still six of them out there, and who knew what piece of luck might attend the dedicated searcher?
Rather a big piece would be needed, Moist had to admit, because four of them were tucked safely away in a little lead box under the floorboards in Moist`s office, Even so, two were out there somewhere, perhaps destroyed. Lost, eaten by snails or still in some unregarded bundle of letters at the back of a drawer.

The book was written after the $5 Blue Triangle was released in the Year of the Prawn, and despite the difference in the face value there are parallels with the stamp being discussed above. This section looks at what might be called the Must Have Discworld Stamp, the $5 Blue Triangle, from its inception.