Porsche speedometers-interchangeable?

[rogerkeith]

I have a 1983 Porsche 911 SC which has a broken electronic VDO speedometer with 4 brass "spade"connectors. The top one is "live", the second is not used, the third is green from the sender unit and the fourth is brown to earth.

I have been offered a replacement unit which only has 3 brass connectors and appears to be an older model. I was wondering if these two units are directly interchangeable or would I be required to "re-wire" the replacement? I do not wish to "try" to see if it works for fear I do damage.