This is a D Sized Dummy Cell. It is used in place of a standard cell to lower overall voltage and maintain the flashlight body length. In effect, you can use one of these in place of a standard cell in a 5D Maglite in order to use the 3-4D XM-L Mag Dropin. In a 6D Maglite you would need two dummy cells.
These are machined from solid bar stock aluminum and covered in heat shrink insulation. They are meant to be functional not beautiful. Some additional trimming of the heat shrink insulation may be necessary for your particular application.