Strange little black box

[behold81]

Before i delve in to a bunck of wiring diagrams i thought i would ask.

In my fuse box i have a small electronic box about the size of a box of matches that has 4 of 8 wires bing used going in to a chocolate block.

Any ideas what it might be? Im guessing its altering the speedo???

On the speedo note when its altered does it change ALL fron km of is the trip still reading incorrectly??