On my new acquired 1985 XJS 3.6 manual, I see a couple of aftermarket things. There is a Speed Control installed (have not found out yet if it works or not).

When I let down the lid below the steering wheel in order to see what the extra added switch would be for, I saw this Electronic Servo packed away. Never seen something like this before. A bunch of wires clips into it.
Is it part of the Speed Control? It is something generic as it can be used for 4, 6 and 8 cilinder engines. It is also marked Manual Transmission which is correct for the car. With 2 adjustment screws and one setting. The setting is for 4 or 6 or 8 cilinders, it was on the 6 cilinder setting which is correct for the car. It shows 250K whatever that may mean.
There is a screw for "system sensitivity adjustment" and one for "centering adjustment" I assume (sticker is damaged).
What does it do? Some kind of ECU measuring the speed and give a signal to the thing under the hood where a cable goes to the throttle? Why would it matter if if is a 4/6/8 cilinder? Why would it matter if it is a manual or auto?

And the extra switch (with the purple wires) - it connects or disconnects a green wire from the main car harness. No idea what it does!

 

Not sure with the 4-6-8 cylinder selection. I guess the system is using an engine RPM signal as well as vehicle speed signal?

Manual trans selection allows input from a clutch switch to disengage the system

Cheers
DD