There are few items in the Koom Valley collectables that come under the category of proof sheet. This first, a Final Proof was available to all from the Emporium. It shows two pairs of stamps and states that the design has Vetinari’s approval - obviously something he did not tell Sam Vimes about!

More interesting though is the Artist Proof sheet. Surrounded by muted but ornate artwork sits one stamp image, without any dwarfs or trolls. These and other elements that went into the final design are included separately, for closer inspection. Indeed, their is an instruction (to Vetinari perhaps) to inspect carefully before approving the design.

Finally, there are a pair of sheets, with one stamp each, which are rather special. They were the Journal readers only offer with issue 7a. Some 600 pairs were produced. They were sold as proof sheets but from a philatelic perspective they are more souvenir sheets. The stamp itself is perforated within the sheet, which at the time and for quite a while was unique. Vince the perforator was flexing his skills. The design of the stamp concentrates on the Koom Valley scene and has a nice minimalist look to it, compared to the more classical original stamps. The valley artwork extends well beyond to show much more of the  valley, revealing its mountainous sides, fallen rocks, icy cold river, and pine trees on the lower slopes. There is one sheet for the Attack of the Dwarves* and one for The attack of the Trolls. They are printed on gummed paper, so theoretically one could tear the stamps out and use them for postage. These remain one of my favourite Discworld Stamp items.
There are some unperforated copies of these in collections.
*It is DWARVES on the sheet, though I have followed Terry`s spelling of DWARFS elsewhere in this article.

For the record, these sheets were A5 sized, so that a pair could be printed on a single piece of A4 paper which would then be neatly chopped into two, before perforating the stamps. Surprisingly, you will just be able to see that in the example on the right both sheets are the trolls attacking version. I would have thought that having one of each per sheet would be the simplest method of ensuring equal numbers of both were printed.