The mere mention of Koom Valley was akin to lighting a touchpaper under Commander Samuel Vimes. He had enough to do without anything inflaming tensions between the dwarfs and the trolls of the city. The last thing he needed was a pair of stamps, issued by the Ankh-Morpork Post Office, to commemorate Koom Valley Day - even if they were impartial by showing dwarfs chasing trolls and trolls chasing dwarfs. This is how the first Koom Valley stamps were introduced in Thud! Luckily, Discworld Stamp Collectors do not have his worries to contend with. In fact, they can build up a Koom Valley themed postal history collection. 

Here, hopefully, is what collectors can look out for to complete a Koom Valley collection. Some are quite common, some less so. It may take a while to complete, but its worth it.