The stainless steel battery extension tube allows the use of two "18650" Li-ion batteries with the Survival Laser stainless steel hosts and parts bundle host options for higher battery capacity, longer total run times and at full output power. However, DO NOT use this battery extension tube with any builds that include a "boost" driver circuit that is limited to a maximum input voltage of 5.5-6V, as you will likely damage or destroy the driver. Also, not for use with the Survival Laser R or any of our tactical flashlights. Brushed finish stainless steel matches the finish of our stainless steel host assemblies. Note: Host and batteries not included.