I got one of these neat little kits for mine a few weeks ago and it works a treat.
The black sponge thing is a self adhesive cushion for the base of the tail cap that lets the batteries get tension for contact and protects it from the metal base so there is no denting or damage to your cells.
I'm actually using it with unprotected cells in a ROP because it's better than chopping the spring but I had to pack the cells out with a magnet at the top and in the middle of the pack so they didn't move, it worked fine without the magnets but I'm just a bit fussy.