The two quart canteen bladder is collapsible and comes with the M1 gas mask compatible cap. It is perfect for those longer trips when more water is needed. The 2 quart bladder canteen is collapsible. The 2 quart bladder canteen is plastic and comes with the M1 pressure cap. The M1 cap is designed to fit the NATO standard 28mm military canteens and has a pressure valve fitting that receives the drinking hose from any NATO standard military gas mask.