Ok - I've learned a little more.
It turns out that the original signal ranges from 0v to 5v (relative to the earth - well, this is a car)
If the voltage exceeds 4.5v, the ecu thinks the turbo is going to explode and sets the car into 'limp home mode' - ie turns off the turbo
Whilst the voltage is less than 4.5v, the ecu uses the information to dictate how much fuel is going to the engine.
So this box of tricks is basically allowing the 0-4.5v signals to go through unchanged
but not letting the voltage exceed 4.5v.
With passive components I can only see this being done with a zener diode - maybe with a few resistors to make it more graceful
But I still do not understand the Mega-ohmed resistances nor the weird flow on the s1<->s2 lines
Could there be a transistor in there? Yikes
So, any help still much appreciated