This case encloses a 50mm x 50mm multi-rotor power distribution board that has mounting holes on 45mm spacing. The specific PDB that this case is made for is available from many sellers in Asia, and a few in the US.
The case protects the PDB electronics from the elements. It is also a good idea to keep multirotor PDBs enclosed for safety. The voltage isn't normally high, but with heavy-gauge wires and lithium-polymer batteries, the instantaneous current potential is enormous. A small metal object shorting the ESC wires or battery wires can cause electrical damage, personal injury, or start a lipo fire that can't be extinguished by normal means.
You can get the power distribution board from Hobby King here.
You also need 4 - M2.5x20mm screws to complete the case.