This is a Camelbak Pouch known as the Max Gear Bottle Pouch, model 1755901000.
External mounting makes this accessory pouch a key addition to any pack configuration. The CamelBak Max Gear Bottle pouch" adapted from the tactical line" offers additional storage for food or tools" and is designed to fit all CamelBak bottles up to 1L. And because it features MOLLE webbing and zips shut" it won't swing around from your pack or lose any of the contents.
Features:
Fully Insulated
Molle Webbing
Zipped Closure
Fits Existing CamelBak Bottles of Up to 1L in Size