The EK-RES X3 - Bottom BLACK is the stock black bottom end cap for EK-RES?X3 series reservoirs. EK-RES X3 - Bottom BLACK is made of quality machined black POM?Acetal material. Enclosed:? - EK-RES X3 - Bottom BLACK acetal end cap - spare sealing gasket (o-ring / OR 52x2.5) This product is compatible with the following EK?reservoirs: - EK-RES?X3 110 - EK-RES?X3 150 -?EK-RES?X3 250 - EK-RES?X3 400