Spedometer question

On the back of my spedometer is some device installed, it looks as a relay, can someone please explain to me what it is.

One of the wires that connet to that device goes to the face disc, i wonder why, i'm about to swap in a km/h spedo from a Fairlady, i need to use the old housing, being that the Fairlady housing is the later type, out from the back of the Fairlady spedometer is two wires, one to the face disc and the other to ? and what is that suppose to be good for.

Chris