This battery magazine from Surefire holds 6 SF123A batteries and fits the M6LT Guadian and older M6 Guardian incadescent handheld flashlights, as well as all "B" option Millenium Forend Weaponlights. The MB20 is an included component of the aforementioned systems, but some users may wish to purchase a back-up unit for quick battery changes in the field. Batteries not included.