My (modern) D2 has some sort of microprocessor control panel attached to it, but, i understand, all this does is switch a couple of wires in the loom to get the unit running. I think the modern units have an ECU within the device as well.
My B4 had a silver box, but, what it had it it, i dont know - it was from the late 70s, so it cant have been too 'custom'. Pretty sure i can see the silver lump in the photos, kind of between the two end caps, under a relay or connector or something.
I should download the manual again, as it has circuit diagrams in it, IIRC.
I dont believe it to be a grands worth. Modern D4s fetch about £300 on ebay. Id guess this is maybe £150 worth, if it were fully working?