This special Bow can be found in the Imperial Palace Armoury,[1] in book 29 The Storms of Chai.
It is a superior quality Bow forged for Imperial Guard officers. It adds +1 to Combat Skill when used in combat. Note this is unusual because Bows are not usually used in fights. One might want to interpret this as providing a +1 bonus to bow shots instead.
This bow is the best Grand Master can get.