If you have converted your 6P, Z2, or C2 to a rechargeable flashlight with the KR2-BK kit, and wish -- or need -- to temporarily use 123A lithium batteries instead of the rechargeable B65 battery, you'll need to drop a Z29 into the flashlight in order to take up the extra space left when the B65 is removed. The Z29 is actually a solid piece of machined aluminum that acts as a simple conductor.