The Battle of Koom Valley commemorative stamps, mentioned in Thud!, were a pair alternatively showing dwarfs chasing trolls and trolls chasing dwarfs across the valley floor. I suspect that it was the Pratchett narrative from the book that directed the central images rather than the stamp design coming first - he was apparently very proactive on the appearance of the stamps in the early days. The artwork is from the hand of Alan Batley, and the overall design by Colin Edwards. They were issued during the Year of the Signifying Frog in sheets of 32 stamps - 16 pairs - eight columns of four stamps. A few imperforate sheet printed on gummed paper exist. There were no sports for this issue, but you may find examples of a paler print suggesting a later print run was required. They were Wincanton perforated. Try for two pairs, with one stamp on the left and on the right.

As you can see the sheet margins are richly decorated with chasing silhouetted figures, and their weapons of choice. That is continued in the BP CE AB logos in the top corners.

I have seen these incorrectly referred to in the past as a Discworld Regional issue. Thud! clearly states they issued by the Ankh-Morpork Post Office, the top text box states Ankh-Morpork, and Vetinari`s bust is prominent. Nobody actually lived in Koom Valley (an old school friend of Lady Sybil lived within a days ride) so there was no need of stamps or a postal service.