It is a 25mm seatpost for 979s. Not really that rare, there are always enough of them for sale at eBay everyday. The one on the Carbones are harder to find at 23mm...unless you want to use a straight post for a BMX bike which shares the same size, it's close to getting to be unobtainium. So never buy a Vitus Carbone frameset unless it includes the seatpost!

Chombi